Had he lounged on his leather bench, thought slightly differently and gone the other way, what would have happened? Well, there was no indication that ministers would have resigned. Sir Nicholas Lyell specifically denied that he would leave office if the Conservatives lost. There would have been howls of outrage but there would also have been a vote of confidence held tonight. And John Major would have had Unionist backing, and won that vote quite easily.
But Allason's single vote was the difference between a Government able to declare the Scott affair closed - whatever newspapers say - and a further period of political crisis, with unpredictable consequences. Mr Allason has not been a particular favourite of government whips. But the Prime Minister owes him a large one, with Worcester sauce and a bag of peanuts.
What makes the victory sweeter for Mr Major is that he does not appear to have given ground to the Ulster Unionists, who had wanted assurances on the proposed voting system for any Northern Ireland "peace vote" in the Spring.
Throughout the day, the public face of parliamentary politics - the Commons debate itself - was being undercut in the minds of knowing MPs by private talks between government ministers and the Northern Irish parties. As a result, though the OUP voted against the Government, Ian Paisley's Democratic Unionists didn't.
All the parties wanted different things from Mr Major. Throughout the evening, rumours had swirled around. But a clear deal last night, tilted to the biggest of those parties, the Official Unionists, might have been the final volley over the corpse of the peace process. We may know tomorrow exactly what proposals Mr Major has come up with in this hideously complicated Irish shuffle diplomacy. But it looks as if he has kept his nerve, as well as winning the vote.
These "great parliamentary occasions" are often a surreal mixture of apparently disconnected events, combining to produce an unpredictable late-night punchline. So it was again last night.
The outward politics of the Scott debate bore almost no relation in logic to the various demands on voting systems and constituencies coming from Northern Ireland.
Yet the two were intimately connected with the result of the vote. It was a typically bizarre proposition; in logic it had about as much meaning as saying that two scarecrows, plus one bowl of custard, equals Iceland.
Labour's disappointment will be all the greater, because they had, any fair observer would agree, comprehensively won the debate on the substance of the Scott Report. Robin Cook's performance was quite simply the best attacking parliamentary performance I have heard in a dozen years of reporting the Commons.
In his command of the detail, his ability to synthesise different strands of Scott's argument and devastating summaries, he outperformed every commentator and other politician in the land. His mockery had ministers and backbenchers squirming and laughing at the same time. His sarcasm and self-confidence silenced a party many of whom had come to barrack; within a few minutes it had become clear that he was able to deal with anything they could throw at him.
It was, in short, a personal triumph of a rare kind. It probably had not the slightest impact on the final vote. On the bare numbers, Parliament has voted to be unconcerned about being deceived, and dealt a blow to the precious doctrine of ministerial accountability.
But Mr Cook's unanswered questions and his scorn matter too. Yesterday's debate will have given the Opposition renewed confidence in taking their arguments to the country. Last night was never going to bring down the Government. But it lost the verdict of the gallery and perhaps, of the nation.
In the short term, this will go quiet. Nothing will follow. Mr Cook's speech will seem to matter about as much as a virgin performance at a student debating club. We in the media have a short attention-span; there is nothing apparently duller than last week's debate, last week's vote. But one day, those same arguments may smoulder and ignite in the minds of uncertain voters. And - who knows? - they may not agree with Rupert Allason.Reuse content