Prime minister's questions sketch By Nick Assinder Political correspondent, BBC News website

That'll teach Gordon Brown to tackle Killer Cable - the man with the withering wit.

Mr Cable has become Mr Brown's nemesis

After his devastating "Mr Bean" put down of the prime minister a couple of weeks ago, the stand-in Lib Dem leader's turn at PMQs has become one of the week's much-anticipated highlights.

And this time he came up trumps with an off-the-cuff thunderbolt that could have knocked the prime minister off his feet - had he been on them.

And Mr Brown had no one to blame but himself.

The prime minister has his approach to David Cameron well worked out, for good or ill.

It is to cast the Tory leader as a lightweight with no substance who would rather concentrate on gossip and personality than substance.

Bit of banter

He did it four or five times this week - just as Mr Cameron sought to paint Mr Brown as indecisive, weak and incompetent.

The tactics in this battle are now drawn and each man is settling into his stride.

Mr Brown does not know how to combat Cable

But Mr Brown is still struggling to know how best to slap down Mr Cable, with whom he previously had a relatively friendly relationship.

Should he go for the tried and tested sneering at the third party that served his predecessors well. Or should he try a bit of banter?

So, when Mr Cable asked him which of the disasters to have battered the government over the past six months would haunt him most over his Christmas sprouts, Mr Brown tried a fresh, pre-prepared put down.

Tongue firmly in cheek, he thanked Mr Cable for his contributions over the past few weeks and suggested that, at the current rate of leadership turnover in the party, he might be back in his stand-in role not long after the next Lib Dem leader has been elected (next week, by the way).

And that was when it all went horribly wrong.

Party poppers

Killer Cable hardly hesitated: "Given his own position, he might not be wise to speculate about leadership elections."

Labour backbenchers looked like they had just opened their Christmas presents to find batteries have not been included.

Mr Brown's smile fell from his face like needles from a Christmas tree on Boxing day (if a Downing Street aide had fed him that line I fear for his or her future).

And the Tory benches joined the Lib Dems in donning their paper hats, pulling their crackers, firing off the party poppers and falling about in delirious festive fun.

Gordon Brown, of course, probably doesn't do Christmas - far too frivolous.

But how he must be wishing for the holiday period to arrive and give him a break, at least from his new nemesis Vincent Cable.