Did the grin — captured as he left Downing Street by the back door — display triumph? Or mask defeat?

The pop psychological profile of the emotionally reticent Chancellor would suggest that not even Mr Brown could explain what prompted that particular expression. But on such split seconds are conclusions drawn. And after a week of extraordinary plotting and unprecedented rancour in which some anti-Blair conspirators believed that they would secure the Prime Minister’s scalp by the weekend, it is Mr Brown’s character, the stuff behind the smile, that is under scrutiny. Charles Clarke was voicing in public what many Labour MPs and sympathisers feel privately when he said that Mr Brown’s smile was not just ill-judged, but a “stupid, stupid thing do to”.

That is a heavy judgment to level against a momentary rearrangement of facial muscles. But its impact rests on the circumstances. With the Prime Minister facing calls for his head, the Government awash with resignations, the Labour Party gripped by turmoil, where was Gordon? Invisible and silent. Until he emerged from two bruising meetings with Mr Blair. Smiling.

If it was a victory smile, it was misplaced. There are no winners from Labour’s week of madness, in which Mr Brown made four angry demands of Mr Blair — that the Prime Minister silence his more outspoken allies, give a timetable for his departure, endorse Mr Brown as his successor and share the reins of power — and got apparently little in return.

Mr Blair, in an early-morning 90-minute encounter, accused Mr Brown of fanning the flames. You have known I am going before next summer for ages, he told his one-time friend — I’ve told you so on numerous occasions. And you could stop the agitating just like that if you chose to do so.

Cabinet ministers and senior party officials performed the role of headline writers, condemning the strike against Mr Blair as a coup and a putsch. It was blackmail, they said, insane and unconstitutional; boss politics worthy of the mobsters from The Sopranos.

Was Mr Brown behind it all? Certainly, there are layers of circumstantial evidence that provide plenty of grist for conspiracy theorists.

The fuse for this week’s fireworks was lit, six long days ago, by a letter signed by 15 members of the 2001 intake of Labour MPs. It demanded an urgent change of leadership and that Mr Blair stand aside. Its combustibility lay in the identity of its two authors, Chris Bryant and Sion Simon, for all the world two loyal Blairites.

There was also a junior minister, Tom Watson, and six parliamentary aides, the unpaid ministerial bag-carriers expected to vote with the Government. It was their resignations on Tuesday that brought home the extent of the difficulties facing Mr Blair.

Those of the 15 who have explained their actions have presented their motives as honourable — concern over opinion polls and uncertainty over Mr Blair’s departure meant that only after a swift handover to Mr Brown could Labour make headway — and their letter as private. But there are sufficient details about the writing of the letter, the recruitment of its signatories and the handling of the fallout to raise questions about this version.

Much concerns the role of Mr Watson, a widely liked former union official and moderniser who has good links with the Chancellor. Six of the signatories are at least partly indebted to Mr Watson for their positions as MPs — he was the Amicus official who helped them, as members of the union, to find safe seats.

Another, Iain Wright, arrived at Westminster after winning the 2004 Hartlepool by-election, a campaign run by Mr Watson, and now shares Mr Watson’s flat in London.

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