The NHS has for the third week dominated a rowdy session of prime minister's question time.

The arrival in the chamber of Paul Marsden, the former Labour MP who crossed the floor to the Liberal Democrats, predictably brought jeers from the Labour benchers.

A more confident Iain Duncan Smith rose to the opposition dispatch box but failed to land the killer blow.

He began with the chancellor's pledge to spend eight per cent of GDP on heath asking whether it was a commitment or a policy.

Blair replied there was no division between he and Gordon Brown saying the government was committed to increasing spending.

"Will he tell us whether he is?" goaded Blair accompanied to cries of "answer" from the backbenches.

Hitting back, Duncan Smith said the devolved areas of the UK were already spending more than the golden figure of eight per cent.

"Perhaps you would like to comment on the fact that in Northern Ireland, Scotland and Wales they are already spending over eight per cent, and yet Scotland has the lowest survival rate for breast cancer and lung cancer of any major European country," he said.

"Doesn't it show that your figure of eight per cent is vacuous and you made it up as you went along. The reality is that you don't have a clue and you don't have a cure for the health service."

Blair was prepared to fight over funding and quoted the Tory leader's description of NHS funding as being "like putting water in a colander". Listing improvements to healthcare in Duncan Smith's constituency he demanded: "Which of that money does he think is like pouring water into a colander?"

Charles Kennedy succeeded in riling the prime minister by raising the issue of redundancies in the post office. "Was the government consulted and if it was what action was taken?" he asked.

Kennedy said there would be rightful dismay in the country if 30,000 are made redundant and the company "did not even consult with the government of the country". He called on Blair to protect rural postal services.

Blair said Kennedy's move was "a classic Lib Dem response" and demanded to hear from Kennedy whether his party would intervene if it was in power.

"It's easy for the third party in British politics to make promises knowing they will never be in power to implement them," he said - quoting the words of the party's newest MP Paul Marsden.