Parliament: The Sketch - PM emerges victorious from shoot-out in the chamber

OFFICIALLY, PRIME Minister's question time ends at 3.30 on the dot. But it is often over well before then. If William Hague rattles off every round in his six-shooter within the first 10 minutes, for example, then the rest of the occasion almost always has something of a desultory air, a duel in which one of the gunslingers has run out of ammunition. Tony Blair can emerge from behind the riddled water-butt he's been using for cover and taunt his opponent with his impotence.

It is not safe to dismiss the occasion entirely - Paddy Ashdown is still out there somewhere with a double-barrel shot-gun after all, and, although he has been aiming wide just recently, he might let a couple of pellets land a little close, just to remind Mr Blair that he's not firing blanks. But what follows will be a mere codicil to the main event.

But yesterday afternoon, the session concluded early for the rarest of reasons. Something had actually happened, something so unusual that experienced journalists headed for their phones well before questions were formally over, confident that nothing could top what had just happened. Unless Mr Blair announced that he was leaving Cherie to set up home with Alastair Campbell, they knew they had their lead.

It began with Mr Hague inviting the Prime Minister to say whether he was happy to see "nearly 100 hereditaries continue to sit in the House of Lords". Mr Blair agreed, a response that bemused some Labour MPs without quite suppressing their Pavlovian reflex to make supportive noises whenever he reaches a full stop. "His party may not be aware of what he is talking about," continued Mr Hague, disappointed by the lack of reaction.

The Government, he explained, had been quietly preparing to cut a deal with the monstrous forces of reaction. In exchange for allowing one in ten of the hereditaries to keep a seat in the Lords, the Tory peers would agree to give up hooliganism.

Quite how the lucky few were to be selected was not clear, nor how long their reprieve would last. Perhaps Mr Blair intends some form of legislative decimation, in which every tenth hereditary would be executed and their bodies propped on the benches to form a posthumous opposition. From my brief experience of the House of Lords it might be quite some time before the subterfuge was discovered.

For the moment, though, it was not the mechanics of the deal that mattered but its mere existence, a fact that Mr Hague clearly believed derailed the Government's argument that this was a matter of constitutional principle. In effect, he was throwing the proposal back in the Prime Minister's face, and choosing the most public and embarrassing moment at which to do it.

But if he thought Mr Blair would be covered with confusion he miscalculated. Yes, Mr Blair confirmed serenely, the Government was prepared to phase out hereditaries in two stages and, what is more, "we have the agreement of the leader of his party in that House".

Labour MPs, completely lost by now, loyally cheered this new concordat with the toffs. Mr Hague's charge had misfired and he could do nothing but plunge the detonator handle again and again in frustration. It did not work - instead of making the Prime Minister look duplicitous, he had left himself looking ill-prepared.

Mr Blair was quick to exploit the opening. "I can't prevent him from engaging in a kamikaze mission", he said genially, after Mr Hague had retreated to his old defensive line about the House of Cronies, "but even his cronies in the House of Lords agree with me".

Mr Blair may yet lose this engagement in the corridors, when Labour MPs have time to reflect on Mr Hague's words about a "huge climb-down", but he unquestionably won it in the chamber.