As so often before in the peace process, we should have our sights firmly fixed on the pendulum. Past experience tells us we should allow for the swing which sees seemingly irresistible pressure on one party suddenly confront the predetermined and apparently immovable position of another.

If a perception preceding the Weston Park summit is of all sides ganging up on Sinn Fein, it is unlikely to survive the next two days of negotiation. Mr Trimble insists there is but one subject for discussion: whether or not the IRA is now ready to begin decommissioning. Mr Martin McGuinness confidently predicts the Ulster Unionist leader will be surprised by a broader approach to the range of issues on Sinn Fein's more comprehensive agenda.

And we may be reasonably sure of this: if Mr Blair and Mr Ahern now expect Mr Gerry Adams to "carry the strain" on decommissioning, they will also be calculating whether Mr Trimble can carry the additional strain on demilitarisation, policing reforms and the "protection of the institutions" against future political threat.

We can probably be certain of one other thing. If the whole exercise translates into another round in the blame game, Sinn Fein strategists will be determined they alone don't end up taking the rap.

At this writing, however, it seems clear Mr Blair and Mr Ahern are not planning for failure. Compare and contrast their approach in the weeks before and after Mr Trimble's resignation as First Minister.

Before, they appeared powerless to prevent political crisis. Since the expiry of Mr Trimble's July 1st deadline, on the other hand, they have summoned all the authority of their offices and put themselves in seeming command. The decision to take the parties away from Northern Ireland to a secure, media-free zone in the British midlands immediately raised the stakes.

At the very least it suggests prime ministerial resolve of the kind deployed at Hillsborough last year to secure Mr Trimble's return to the power-sharing Executive in return for that IRA promise to put weapons permanently and verifiably beyond use.

Mr Blair and Mr Ahern would hardly have made such elaborate preparation for another fruitless canter across uncompromising terrain.

And for all Dr John Reid's insistence that he wouldn't want to raise expectations, the truth is they will be soaring as the helicopter takes Mr Blair and Mr Ahern from Chequers to Shropshire early this afternoon.

Word in Dublin has it that the IRA has cut Sinn Fein some slack.

Downing Street declines to say if Mr Blair has received similar security advice. Some insiders say the Prime Minister would find such hard to evaluate in any event.

Remember the reported "seismic shift" in the republican approach to the arms question two years ago this month?

Insofar as there is expectation of movement, it seems firmly attached to the future of those arms dumps already opened to inspection. It is attended, of course, by exactly the kind of "rational" arguments which previously decreed that the Provisionals would "have to do something this time". Think back to the Mitchell Review. Or to the first of the two Hillsborough negotiations.

Or, even, to the anticipation which preceded President Clinton's last official visit to Northern Ireland in December. Recall the certainty of the predictions that they would "do it for Bill", and realise that the republican movement has consistently acted according to its own rationale.

As one senior source observes: "It's all very well people talking about the pressure building on the republican leadership. But these guys simply don't know pressure as it applies to conventional politicians."

Mr Blair and Mr Ahern will be hoping to dispel that particular myth. And the "rational" argument underpinning hope this time around seems clear. Mr Adams's electoral strategy has proved popular with the republican constituency. The punters have bought the leadership's "take" on the Belfast Agreement as a transitional route to Irish unity. The next quantum leap (it is presumed) is to see Sinn Fein ministers in government on both sides of the Border.

To realise that ambition, so the argument goes, IRA decommissioning will have to be well advanced, for the assumption is that "democratic" Ireland would not have it otherwise.

Mr Trimble knows that politics at the sharp end is an unsentimental business. He harbours no illusions that republicans will act to save him. His certainty that they will, in the end, decommission - and he has been convinced of this throughout - is rooted in the belief that Sinn Fein's electoral self interest demands it.

So will they? At this point? This very week, to spare Mr Blair and Mr Ahern the agonising choice between a second suspension and a possible last throw of the dice in fresh Assembly elections? And will any move now be enough for Mr Trimble's party? Or will the post-general election reality prove to be that more is required in a situation where less is on offer?

Leading Ulster Unionists last night moved to increase the pressure on Mr Trimble. They insisted privately that a "one-off gesture" by the IRA - for example, concreting dumps already deemed to have been compromised by inspection - would not suffice.

Invoking the questions previously posed by Mr Seamus Mallon, party sources indicated they would need details of "the how and the when" of subsequent decommissioning. Specifically, that would appear to translate into a demand for upfront agreement on modalities and future timetables.

Mr Adams has consistently said republicans will not comply with a timetable set by either Mr Trimble or the British government. However, Mr Trimble has publicly voiced the fear that republicans might do what he would regard as "the minimum" necessary to make life difficult for the unionists.

There is no evidence to support theories that the Blair-Trimble relationship is cooling. Indeed, the view in both capitals seems to be that Sinn Fein would, on balance, prefer to see Mr Trimble survive.

But if republicans offer movement Mr Trimble's colleagues deem insufficient? Would Mr Blair call the whole game off on the say-so of Mr Jeffrey Donaldson or Mr David Burnside? Or might the two governments then hope to realise the coercive effect on Mr Trimble's Stormont colleagues of the Taoiseach's insistence that fresh Assembly elections would be preferable to a second suspension?

No one should assume the pendulum will remain static.

Frank Millar is London Editor of The Irish Times