Last night David Burnside was due to chew sausages and beef burgers at a barbecue in Randalstown, Co Antrim, with party officers and ordinary Ulster Unionists from the South Antrim constituency.

It was a get-to-know-you exercise ahead of the Westminster by-election expected in the autumn. Mr Burnside is not that easy to know. One hears conflicting views and descriptions of him.

"A class act, but a cold fish" was a terse character analysis from an Ulster Unionist who wasn't sure how to take him. As well as astute, shrewd, tough, ruthless and uncompromising, the adjectives "unfathomable" and "deep" are also tossed about.

David Burnside's selection as the Ulster Unionist Party's candidate to challenge the Rev Willie McCrea in the by-election battle had an absolute inevitability about it.

"He has been the next big thing in unionism for a long time," one party colleague said of him. His former Vanguard leader and mentor of the 1970s, William Craig, told The Irish Times this week that what surprised him most about David Burnside was that he had been outside politics for so long.

Going back further to his boyhood and teenage years, he confided to one of his schoolmates at Coleraine Academical Institution that his ambition was to be prime minister of Northern Ireland some day. No more unionist prime ministers now, but something for the First Minister, David Trimble, to dwell on.

Something to mull over also for aspirant leader Jeffrey Donaldson, and for John Taylor, who has not totally abandoned his ambition to be UUP chief. Both MPs are reasonably close to Mr Burnside. In fact, Mr Burnside supported Mr Taylor in his unsuccessful bid for the leadership in 1995.

Vanguard may have slowly expired in the late 1970s, but its right-wing legacy lives on in a paradoxically liberal way. Mr Trimble and Sir Reg Empey, during their time in that organisation with Mr Craig and Mr Burnside, helped to destroy the Sunningdale power-sharing experiment of 1974.

Now they are central to this latest "Sunningdale for slow learners" power-sharing project. Mr Craig is long retired, but his former protege has entered the frame. Mr Burnside, at 48 still a relatively young man, is favourite to win the South Antrim byelection despite the serious challenge from Mr McCrea of the DUP.

Then what? Is Mr Burnside's plan to also adopt a moderate mantle, co-operate fully with Mr Trimble and work the Belfast Agreement? Or is he scheming to usurp the First Minister?

Such questions were being asked within unionism after Mr Burnside defeated Mr Trimble's chief-of-staff, David Campbell, for the South Antrim nomination last Monday. So what are his ambitions?

He insists that he is no threat to Mr Trimble. But he would say that, wouldn't he? There are faint echoes here of the former taoiseach, Charles Haughey, who after the Arms Trial of 1970 had to spend time out in the political sticks before returning triumphantly as Fianna Fail leader and Taoiseach in 1979.

Indeed, the late 1970s was the period when Mr Burnside effectively severed his contacts with the far-right Vanguard organisation and transferred from politics into the world of public relations.

But even through the 1980s and 1990s, when he was away from the unionist political centre, his influential associates, such as Lord Cranbourne, were certain that eventually he would enter Westminster. Unlike Charles Haughey, he never had to work the rubber chicken circuit to make his comeback.

The sophisticated circles of London rather than dreary political meetings in isolated Orange halls and community centres around Northern Ireland was where he made his mark. That's where he won allies for unionism in the Tory Party and the media, and influenced them.

When leader-writers or commentators for the Sunday Telegraph, Daily Telegraph or Spectator turned to advocating the unionist case, they often sought guidance from Mr Burnside.

He's a successful public relations consultant with his own business in London. The one blot on his career was when, with British Airways, he suffered the indignity of seeing Richard Branson's Virgin Atlantic get the better of BA when Branson alleged a "dirty tricks" campaign against BA. While his own department was not linked to the campaign against Virgin, Mr Burnside didn't - and doesn't - like to be on losing sides.

He married for the second time about 18 months ago and commutes between his homes in London and Co Antrim. His interests are shooting and fishing and he has a farm near Ballymoney, pursuits that should go down well with the rural folk of South Antrim.

Unionists, like many others, can't quite figure him. "He won't be able to work Ballyclare market the way Willie McCrea will", said one Ulster Unionist unimpressed with his country interests.

"Don't worry about Ballyclare market, he has enough experience in the PR world to get his point across at Ballyclare market, or anywhere else in South Antrim, or beyond for that matter", responded an admirer.

Mr Craig would make no comment this week about Mr Burnside running for the Ulster Unionist Party "because I don't support the Ulster Unionist Party". But he remembers him as a very efficient press officer for Vanguard. "He was quick, he could see what was political, what was important, what the reality was, and he knew how to express himself", Mr Craig said.

What is crucial for Mr Trimble and the UUP is how Mr Burnside applies this genius. There is no doubting that were he so minded he could make great mischief for the First Minister. As was evident on Monday night, he is expert at playing it both ways, calling for unity, appealing to the Yes and No camps at the same time, leaving each side impressed but slightly confused as to where his allegiance lies.

Mr Trimble is obviously vulnerable. He must negotiate a very difficult strategic path by following what has been dubbed a policy of "creative ambivalence". It isn't easy.

He is dependent on the IRA quickly doing what it said it would do in initiating a confidence-building measure on arms. After every hurdle is surmounted, another one looms a furlong ahead for the First Minister, whether in the shape of policing, decommissioning, prisoner releases, parades, Drumcree, flags and symbols. But he's keeping faith with the agreement.

This is where Mr Burnside, an expert strategist, must make a personal strategic decision. As a true ally of Mr Trimble, he could deflect or muffle the constant sniping of the No camp, and even bring some of the anti-agreement bloc back to the middle ground.

His public relations expertise could also be vital in modernising the UUP. His tough-guy demeanour could stiffen the resolve of party members and beef up their confidence again. Ian Paisley, Peter Robinson and Willie McCrea know he will be no soft touch for the DUP.

Trimble and Burnside could even follow a soft-cop-tough-cop routine to the benefit of the party, while also conveying the message to the old guard that the unionist monolith is a thing of the past.

Mr Burnside could be a formidable figure in Ulster Unionism and in Northern Ireland politics generally. He could be the natural successor to Mr Trimble in the longer term.

But if his ambitions are more immediate he could choose to overplay the decommissioning and RUC issues, and he could decide to shift further to the No camp than to the Yes bloc. That could lead to leadership of the UUP - or a fragmented version of it - in the shorter term.

But it would hardly serve the grander ambition of the Belfast Agreement of achieving a historic nationalist-unionist political consensus in Northern Ireland.

It's Mr Burnside's call.