Time flies when you're having fun, they say. But for 30 precious minutes this week it actually seemed to move at supersonic speed. The reason was Peep Show, which returned to Channel 4 on Wednesday for a ninth six-part series and was so good and welcome and funny that it seemed to pass in a flash.

As well as being the best British sitcom of the 21st century - and I'll defend that claim against all-comers: just pick up the phone - Peep Show's return after three years is doubly welcome because that hiatus was supposed to be of the permanent variety.

Writers Sam Bain and Jesse Armstrong told us so prior to the screening of series eight in 2012. “We don't want the standards to drop, we want to keep the stories fresh, and there are only a finite amount of stories you can tell about any character,” Bain said. “We don't want to be remembered as the guys who let it drift into mediocrity. We'd rather go out on a high.”

They didn't and they undoubtedly will.

So why is Peep Show so great? Certainly not because of its dazzling originality of plot. It's about two old college friends, Jeremy (Robert Webb) and Mark (David Mitchell), who share a flat together, so the basic premise is straight out of The Odd Couple. Jeremy also shares DNA with Withnail, the character played by Richard E Grant in Bruce Robinson's cult 1987 film Withnail And I. He's also like every toddler you've ever met: self-centred, temperamental and, well, childish.

Both he and Mark are equally hopeless at the big things that matter - diet, girls, jobs - so there are elements too of the Simon Pegg/Nick Frost dynamic in late 1990s sitcom Spaced, which pre-dates Peep Show by several years. There are other comparisons: where Spaced also had national treasure Jessica Stevenson in its cast and featured an oddball conceptual artist called Brian Topp, Peep Show has national treasure Olivia Colman and an oddball crack addict called Super Hans (played with leering gusto by Matt King).

Instead, it's something terribly old-fashioned which makes Peep Show so brilliant: the scripts, written not by a huge team of pens-for-hire as in America, but by two friends who sit side-by-side in an office and knock ideas off each other until one of them comes up with a line like: “You realise tinned food is just for crack-heads and wars?”. Or: “Maybe he's on acid or watched a whole Jeremy Kyle”. Or: “I suppose doing things you hate is just the price you pay to avoid loneliness”.

But allied to the scripts is something else, an innovative structure blending point of view shots with voice-overs detailing Mark and Jeremy's thought processes. That disparity between the words that come out of their mouths, and the things that run through their heads adds to the fun. If you can call social dysfunction, self-obsession and moral nihilism fun.

Where Peep Show is concerned, we can. The only bad news is that this series really is the last. Come 10.30pm on December 16, we'll finally have to say goodbye forever.