Tony Blair reckons that the Conservatives' recent commission suggesting a future Tory government could cut taxes to the tune of £21bn has given him a valuable weapon during his weekly joust with David Cameron.

Having spent so long studiously avoiding anything that Labour could pin him down on Cameron now finds that whatever area he criticises the government on the prime minister can retort that things would be worse under a service-slashing Tory administration.

Or as Blair put it in answer to a friendly backbench enquiry: "We will not make uncosted commitments to billions of pounds worth of tax cuts that can only be afforded by depriving the poorest in society of the help they are currently getting."

And so it was when Cameron used the chief inspector of prisons' claim that the youth justice system was reaching "breaking point" to try and attack Blair over law and order, the prime minister was able to turn it round.

Blair said that the Tory leader was unable to commit to the 8,000 extra prison places Labour was funding because of his "tax cutting policy".

After digressing on the NHS, Cameron said: "Now, prime minister back to prison, actually that's got a certain ring to it." A reference to the rumour that Scotland Yard are about to question Blair about the loans for peerages scandal.

Having brought up the health service Blair laid into his opposite number much to the delight of Labour backbenchers who are now enthusiastically backing the prime minister at these encounters.

Trying out a new soundbite Cameron concluded: "It doesn't matter who's in charge - Blair/Brown, Brown/Blair - this country isn't safe under Labour."

Blair hit back with a soundbite of his own: "The truth is he talks tough but he votes soft."

When Cameron asked about a climate change bill he got a similar response with Blair saying: "I'm surprised I described his proposals as a 'trifle dodgy', I think plain dodgy is more accurate."

The prime minister then provoked Tory hilarity by suggesting the leader of the Opposition and his shadow chancellor were divided. The idea that Blair and Brown are role models of co-operation between a leader and his economic spokesman did seem a bit rich.

Sir Menzies Campbell then asked about extraditing the US soldiers responsible for the unlawful killing of ITN reporter Terry Lloyd in Iraq.

In doing so the barrister and former foreign affairs spokesman stuck resolutely to his comfort zone issues leaving domestic areas to the Tories as he has done repeatedly on these occasions.

Claire Curtis-Thomas then surprised the PM by saying: "At this moment I have 100 rather attractive, naked men outside my front door." She was of course referring to Anthony Gormley's exhibition on a Sefton beach threatened by the local Conservative council.

Blair said: "It is a lot better than what I have waiting outside my door, which is the media every morning." Gesturing to the press benches he added: "I suppose we should be grateful they are clothed."

The verdict

Tony Blair: 7/10 - Competently despatched Cameron by rubbishing tax plans

David Cameron: 6/10 - His own tax commission has created a rod which Blair is able to beat him with on public services

Sir Menzies Campbell: 5/10 - Worthy, but safe, probing of UK/US extradition arrangements