Peep Show 6, C4

David Mitchell’s smarty-pants TV panel show ubiquity – over-exposure even by Stephen Fry’s standards - may have started eroding the goodwill built up over five series of Peep Show, but all is forgiven once he’s safely re-garbed in the horribly plausible, drab office-wear of nerdish flatmate Mark. It’s difficult to imagine any other comic actor giving quite the same defeated peevishness to the line: “A new boiler... surely the least enjoyable way to spend a thousand pounds.”

The gag belongs of course not to Mitchell but to Jesse Armstrong and Sam Bain, the writers who have worked miracles to keep their creation so fresh and full of vim right into its sixth series. The Old Guys, their enjoyable BBC1 sitcom starring Roger Lloyd-Pack and Clive Swift as kind of OAP versions of Mark and Jeremy, seems to have been quietly shelved after one series, but Peep Show just keeps on going.

The beautifully simple USP, that we hear what the two main characters are thinking, is the steel girder in this durable edifice of mirth, making for a winning comedy of mendacity - unspoken or subconsciously leaked “bad thoughts” being the stock-in-trade of many of the best sitcoms of recent years, from The Office to Curb Your Enthusiasm. And hats off to Armstrong and Bain for avoiding the self-indulgence that so often creeps into great sitcoms of this vintage. As soon as we spot the first starry guest appearance (Simon Pegg, perhaps, or some hitherto unsuspected American fan such as Will Ferrell) we’ll know that all is doomed.

The main arc of the new series involves Mark and Jeremy’s fear of parenthood and last night’s second episode found them poised for the results of the paternity test that will establish which of them is the father of Sophie’s baby – the mother-to-be (the excellent Olivia Colman) having dropped by earlier to conduct a DNA swab from inside their cheeks. Jeremy  (Robert Webb, who doesn't appear on TV panel shows but thoughtful documentaries about T S Eliot) feared that the genetic material would have to come from somewhere lower down - characteristically for somebody who tends to think with this particular body-part.

Meanwhile the flatmates were enjoying their last moments of freedom by chasing the women of their dreams, Mark’s siren being ex-work colleague Dobby, re-encountered during a game of the paintball-style "Laser Commando" (they later settled down to watch golf on TV). Jeremy is pursuing gorgeous new Russian neighbour Elena, an enigmatic dope-dealing legal secretary with a taste for poetry.

The pivotal comic mechanism was Jeremy claiming that the baby was his in order to impress Elena, only for Sophie to telephone Jeremy to say that Mark was the father. Jeremy then had to pretend to Mark that Sophie had said that he, Jeremy, was the father, and so on, Mark’s ecstatic relief at being unencumbered with offspring turning to dust, his Dobby-induced erection to flaccid worries about schools, affordable clothing and the MMR jab.

Classic farce faultlessly executed. There is no compelling reason for Bain and Armstrong to ever stop making Peep Show unless David Mitchell does one panel show too many, tries to bite the smirk off Jimmy Carr’s face off and gets carted off to Broadmoor. I could happily listen to Mark and Jeremy’s thoughts about each other until Sophie’s child has been put through university, and the flatmates become eligible for their bus passes. But then it would be called The Old Guys, and would probably get quietly axed.