Brown knew that time was running out. You could hear it in his voice

It wasn't just the Prime Minister who flunked the final test. The format did too

The BBC's title sequence for the last of the prime ministerial debate showed an animated map of the country with circular blast ripples pulsing out from various locations. None of them appeared to be centred on Rochdale though – and in the event, Wednesday's humiliation barely registered in what followed, barring one last trill from the Prime Minister's five-act opera of remorse.

"As you saw yesterday," he said in his opening sentence, "I don't get all of it right... but I do know how to run an economy". Pre-match rumour had hinted there would be a self-deprecating joke from the Prime Minister to lance the boil. Was that it? And if so which half of the sentence was meant to be the funny bit?

The backdrop this time round was a gently throbbing heat map of the Houses of Parliament – the stage a dark mirror that looked like an arena for a Pasodoble. But the rules were still as firmly in place and the feet as firmly planted. David Dimbleby – the last of the professional broadcasters to attempt to pry the format apart and sweat the candidates, tried harder than his two predecessors – actually pitching an inquisitorial question at Nick Clegg about post-election alliances. But it wasn't answered and for the most part he had to make do with the faintly threatening jocularity he carries over from Question Time.

The first effective blow went to Cameron, translating Brown's "six billion taken out of the economy" into terms that make it sound as manageable as downsizing your morning coffee. "One pound in every hundred the government spends", he said – as he had done in the first debate. If you can't make cuts that modest, the thinking presumably goes, what price your claim to economic competence.

Gordon Brown shook his head, more in sorrow than in anger, and you imagined the inner man thinking longingly about stabbing some upholstery. Instead he took it out on Clegg.

"I will never form an alliance with a Conservative government that cuts child tax credits," he promised the Liberal Democrat leader resoundingly, the one pledge made last night that we can be pretty sure is going to be kept. That was where Dimbleby tried to press Clegg – the old instincts overriding his acceptance of the protocol – and suggesting, incidentally, that the debates might benefit from looser reins the next time round. We can take it as read that they all want banks to lend more money. What we don't know is how precisely they aim to do that.

An unleashed moderator might have been able to cut the pious boilerplate and drive them to specifics more quickly. And mere humanity would suggest that the audience should be unmuzzled and allowed some decorous reaction. If it's exasperating to watch when you shout at the screen imagine how tough it is when you can't even groan audibly.

An audience member asked a question about immigration and ignoring the views of the ordinary voters, and you wondered whether Gordon's gaffe might finally get a bit of limelight. Dimbleby, visibly twitching, could only repeat the question with meaningful stresses, rather than drawing out its subtext explicitly and looping it around the Prime Minister's neck, as he clearly longed to do. Brown – as if forestalling a low blow – said "I agree with David" and turned the fire on Clegg. And this was the one moment where sparks flew: the Liberal Democrat visibly flustered at attacks from both flanks on the idea of an amnesty for illegal immigrants.

"Let's just save time and assume that every time you talk about our policy you get it wrong," he said to Cameron.

Then it was time for the last of their direct addresses. "If you vote Labour it's more of the same. If you vote Liberal Democrat you get uncertainty," said Cameron. "This time you can make the difference," said Clegg, making his last big pitch for the uncertainty vote. And Brown flustered, eyes darting away from us at home to the concealed timer the candidates can see. I think he'd spotted that his time was running out – and you could hear it in his voice.