On the surface, it might seem that Thin Lizzy could have their eyes and hearts and minds on only one thing – the present. For a start they've just delivered what's commonly regarded as their definitive recorded statement in the brilliant Live And Dangerous double album set. Not only that, but the critical acclaim which greeted the work has been more than borne out by the immediate and, so far, sustained commercial success it's achieved.

Live And Dangerous has been in the top three in Britain - and don't forget we're talking about a double album - for the six weeks since its initial showing, being denied the number one spot only by the sales-monster of the decade Saturday Night Fever; it's never easy to compete against an album that's got a film to market it and, what's more, a craze. And still Lizzy are in there with a fighting chance.

The same indeed is the case in Ireland, with extremely healthy figures to date suggesting that, in the long run, it can outsell their biggest money spinner so far, Jailbreak. Again, they've been No. 2 in the Hot Press charts for a month with only (guess who?) the Stigwood gang holding them out.

On the live front, there was the successful negotiation of the Wembley Empire Pool hurdle, with two packed houses there finally establishing the band, with no remaining grounds for reasonable dispute or logical argument to the contrary, among the Major League as regards drawing power.

With that kind of momentum built up and an American tour imminent – almost certainly with the fresh promotional thrust of a new label, Warner Bros. behind them – there would seem to be little room for pausing, taking a breath or making any other nod in the direction of self-analysis.

And yet, amid all this high-powered achievement, that's precisely what's happening within Thin Lizzy. The rationale, of course, is simple. Stagnation kills. The way to ensure survival on a creative level is to keep one step ahead of the play, to realise when you've been doing material long enough, to anticipate and therefore forestall the point at which it begins to fossilise right there in front of everyone on stage.

That hasn't happened with Lizzy, nor is it likely to, because they've been putting their collective brain to work on the problem in time. They've been doing largely the same set for about two years now - which is why Live And Dangerous ideally represents a watershed for the band – and now they're ready to move on.

Where to? That's the question.

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Phil Lynott has already gone on record as saying that he sees the group concentrating even more exclusively on the harder, rockier aspect of their personality. Lynott the romantic will be put on ice, more or less, for group purposes – only to be given freer rein in the shape of Phil Lynott, solo artist.

There's a plan to release 'Jamaican Rum' as the A-Side of the first solo single from Phil – a track that bubbles along with acoustics prominent in a calypso-type arrangement that oozes romance.

About the shape of things to come in Lizzy, Phil is definite: "Yeah, the music is gonna get more aggressive. You can get that effect maybe by using open chords rather than bar chords – that type of thing."

Previously, Lizzy's fire-power was always tempered by moments of sheer tenderness. Now it'll be unremitting? 'Are You Ready To Rock?' could become the new signature tune when the boys are back in town.

But Brian Downey sees things differently. Indeed talking to the Lizzy drummer offers a new perspective on the inner workings of a band that'll never be noted for its lack of internal 'dynamics'. Unity and mutual identification may indeed be developing both within and about Thin Lizzy, but the band will always be composed of four different elements, between which there are inevitable occasions of conflict. In that scheme of things, Downey is indubitably Earth.

To some extent it's as if the interview offered him an opportunity to reflect on what misgivings he has about the group's current musical standing. There's no hint of disillusion – his belief in the collective animal is still unflinching – but he certainly feels very strongly that the time has come to move on, pronto.

"We're going to the States," he opens, "so that should be the start of a whole new Thin Lizzy. This last year is the first year we slackened off a bit. Since we left Ireland in '70/'71, we've been working non-stop but now we've got a bit of time to get our thoughts together, see what we're going to do. Maybe there'll be a change of direction. The live album is our biggest yet – it's the ultimate we can do, so now we've to put new stuff into the set."

Which is where things get interesting. Downey just can't see what Phil can mean when he suggests Lizzy's stance will become even harder. "I don't see why he says more hard rock," Brian Downey reflects. "We're doing more now than we ever did. We only do one slow song, 'Still In Love With You', and 'Southbound' is medium tempo. Apart from that, the rest of the songs are hard rock, straight down the line. I can't speak for Phil but all I know is that we're playing very well on stage at the moment. It takes it out of me anyway. That means albums."

Which brings us to the likely source of whatever confusion exists within the Lizzy collective at present. Live And Dangerous is definitive Thin Lizzy, but very few people seemed to feel similarly about their last studio album, Bad Reputation, which I found immensely rewarding incidentally, or indeed about Johnny The Fox. If there is a minor crisis within Lizzy, it's because there was a feeling abroad that neither of their last two studio albums fully cut it. The pressure is on to really deliver the goods next time out – but first you've got to decide what was lacking.

Of Bad Reputation, Brian comments, with remarkable candour, "I thought there were a couple of numbers that were really duff on it. The sound was very good but we just didn't have enough time to get the numbers together."

They recorded the album in Canada and there, once they were in the particular studio they'd chosen, they were committed to getting it finished. "I found that after a while," Brian Downey recalls, "when a few of the numbers weren't happening, you couldn't say 'Stop! Let's forget it and go back home and start again'."

The pressure was already on the band at that stage anyway: "Everyone said that Johnny The Fox was a flop, which is totally untrue, I've got a gold record there for it – it went gold in the States, but over a period of time. But it didn't get into the charts, so people automatically say it was a bum album, so we were going over saying we'd have to do something better than Johnny The Fox. But we just weren't ready for the short space of time we had, which was three to four weeks recording. We needed more."

In fact time can be a crucial factor in defining the ultimate shape - and success - of an album. A lot depends on a band's other commitments.

"Jailbreak was the culmination of a lot of work over a long stretch of time. It wasn't an album that was written over a period of a few weeks or months... it took nine months to a year for the whole thing to fit into place, whereas Johnny The Fox was written over a period of three to four months. We've seemed to have less and less time for the albums since Jailbreak, because we were more successful with that album."

The situation isn't without its layers of irony.

Other question marks are raised around the interpretation of recent studio activities undertaken by Downey and Lynott, along with Gary Moore. Brian offers: "We've been in the studio putting down some new material. Originally, it was for Phil's solo album. We've done about five or six tracks, one or two of which he wants to use as a solo single. The rest of the stuff may be on Thin Lizzy's album."

It becomes clear that that's the way Brian would like to see it: "If Phil wants to use some stuff on his own with Gary, it's up to him but it's easy to take Gary off and put Brian on. It's a new direction for Thin Lizzy, maybe. One or two of them are more laid-back: they're not out and out rockers."

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It may be that there's an element of conflict between Phil's desire to get a solo album out and the band's need to make their next LP a classic. On this point, Phil is categorical: "Everything revolves around Lizzy. The band comes first every time."

There may also be some pressure from other members of the band to get more of their songs on Lizzy albums - something which, on the evidence he has to hand, Downey wouldn't particularly welcome. His enormous respect for Phil Lynott as a songwriter comes through in his insistence that the latter seldom if ever fails to come up with the goods.

"In the long run, for lyrics especially, Phil is the main man. You need somebody like that - otherwise it can be touch and go about getting the material together at all. The band has reached a high level, so the music has to stay at that high level. Anything lower than this won't do. There's a lot of stuff thrown out of the band because it's not good enough and Phil really comes up with the goods."

But ultimately, it seems likely that two distinct, though hardly radically different, visions of what Lizzy are – or should be – are at work. Phil comments: "It's harder for Brian to accept that the music might get harder and heavier because he likes to play across a broader range of material, as a drummer."

Brian: "With this band, it's a very democratic set-up. If you don't like something that's happening, you can just go down to the office and say, right, let's talk about it."

Again, on Lizzy's music, he ruminates: "A lot of people describe us as Heavy Metal, which surprises me. I wouldn't like to see the band get into a position where it had to depend on hard rock material to sell.

"Maybe if we slowed down a bit on stage. Interspersed the hard rock material with other materials, the way we used to . . . When it started, Lizzy wasn't a hard rock band."

So how does he feel about the way things have developed with Lizzy since the band's inception?

"I sometimes wonder about that. I've been in the band since 1970, so that by now, I just take it as it comes. In the beginning, with Eric and that, it was fun – if you got a tour, you were lucky. Then we got fairly big – and Gary Moore came in. I took it then that it was either now or never – but it didn't happen. We talked then about material, what way to approach it – we really didn't come up with any answers. It's often just being in the right place at the right time with music. There are hundreds of bands around with very, very good musicians, writing good material, who never make it. It took us five years to do anything."

But the point with Lizzy surely is that there was a core to the band enterprise encompassing Lynott, Downey and joint managers the Chrises, Morrison and O'Donnell, who had the vision as well as the guts to stick it out through thick and thin.

Brian: "Management is 50%. If you have a good manager and can trust him and depend on him, it gives you more incentive to get out there and play – whereas if you've a manager who isn't 100%, you get the feeling you're being ripped off, at which point it isn't worth it."

In fact Lizzy have weathered the successive storms with which Eric Bell, Gary Moore, Brian Robertson and Gary Moore again (though it was never official that he was back in Lizzy full-time) split so successfully, that it's possible to project that there's the kernel of something permanent there. What about ten years time?

"I don't know what I'll be doing in ten years. I tell you, though, in 1970, I didn't see Thin Lizzy still being together in '78 - put it that way. I imagined that the longest a band could be together was three years – but with Thin Lizzy, there are so many different influences and styles that you can go for, that I can see the band being around for a long time. It's hard to know. Nobody gave the Stones 15 or 16 years. But I wouldn't like to predict. The band could break up in 10 weeks."

At which point Downey predictably breaks into laughter.

There is one other slight bone of contention for Downey. It comes up by accident as he discusses what rock drumming is all about. Coming from a man who is undoubtedly one of the finest in the business, his words are worth weighing anyway.

"Just holding the beat there – that's what rock 'n' roll is all about. Holding the beat there and putting something in when it's needed. Holding back. Staying out. Subtle things - putting touches in where people wouldn't expect them. Doing nice things with the high-hat here and there. Or cymbal crashes."

Solid and tasty – that's Downey. Just listen to Live And Dangerous for one of the meatiest, subtlest – and ultimately most inspiring – drum performances you're likely to find. Like many masters, Downey minimises, totally unselfconsciously, what it takes.

"There's nothing much to playing drums. People say to me, 'How do you keep producing the energy? How do you play that solo?'... " – and here comes the bone of contention – "Well, I'm sick of playing solos and putting everything in, in one go. I think it's impressive for the audience, the fans like it – but playing it over two years, it gets boring.

"I'd like to drop the solo. There's been talk about it. It might seem strange, but I'd also like to give the double kit a rest on stage, use one bass drum. The basics. But people didn't agree, so I'm back to playing the two."

It must be weird for Downey feeling that, since 'Sha La La' is one of the few tracks on Live And Dangerous which was consistently put down by the press. But Phil Lynott dismisses that criticism.

He comments: "Critics generally don't buy albums, so they don't like to see people show off – they take vinyl too seriously sometimes. When you've got a drummer like Downey, you've got to give him a spot in the show 'cos you know people are gonna say 'Wow, I didn't know he was that good'. Brian Downey is one of the best – and one of the most under-rated drummers around."

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Whatever way you look at it, it's an issue riddled with contradictions. Brian Downey himself doesn't by any means ridicule it but he's aware enough to realise that it's the kind of thing people tend to like for all the wrong reasons.

"I've tried to do a little more with it. It's not a stop and start solo – it builds. For me it's either the showstopper, the focal point, or it's a complete flop. There are some nights, if the band is playing badly and I come in with a stormer of a solo, it can lift the whole night. But it seems to have stood still. It hasn't progressed. A lot of people come up to me and say, that's the best solo they've ever seen and I think I've played a load of shit."

In the end, he remarks, laughing almost ruefully: "I think that solo will be in there for a while."

So the picture is of questions being asked within the band. But far from being unhealthy, that in itself must stand as a portent for the future vitality of Thin Lizzy. Talking to individual members of the group, you get the feeling that none of them are willing to stand still, that the presuppositions on which many successful bands operate will be thrown down for examination and torn apart if necessary.

No fear of flab here.

Thin Lizzy are at the height of their powers – it's almost a cliche by now that they're the finest hard rock band extant – which is something the tenor of this article shouldn't obscure. At the heart of the whole operation, there's an honesty and clarity of purpose that endures irrespective. Somehow Brian Downey seems like the ideal person to put words on it – but it applies equally, I'm sure, to the whole band.

"I just get up there and play," he says. "It doesn't matter a shit how much you're earning. You just get out there and say, right, this is me playing the drums with Thin Lizzy and enjoying myself. You ask yourself 'What did I start in the business for in the first place?' To enjoy myself. To play the drums and play well and to give the audiences some pleasure, get them to enjoy themselves.

"I starved for four years, but that didn't discourage me. I'm either a musician or a brick layer. If I'm gonna do it, I'm gonna do it well. Give it all I have."

Nothing less given. Nothing more asked. And from that basic position, you can learn the balance to ride the changes. Thin Lizzy are on the crest of a wave.

I don't see it breaking.