Bill Grundy sits swinging in a swivel chair, swigging from a hip flask and reflecting on the wreckage of his life. If you've heard of Grundy at all, it'll be because of the Sex Pistols: he's the hapless besuited interviewer who inexplicably goaded them into a flurry of obscenities, live on 1970's teatime TV. The episode cost him his job and his career, and we see him now – deliciously, hilariously – reduced to presenting a show called Close Up On Castles, a lunchtime filler which is clearly just as pointless as it sounds. We know that no-one's watching any more; and crucially, so does he.

Grundy's repeated attempts to record a piece to camera, in the face of interruptions such as a passing coach tour and a urinating dog, perfectly reflect the decline of his career; or as he phrases it, "the toilet into which it has plopped". Alex Dee offers a hilarious portrayal of the one-time star reporter, punctuating exquisitely acerbic lines with perfectly-timed eye-rolls, letting the briefest of pauses or variations in his delivery signal the crushing disappointment that Grundy's life has become.

His humorous disdain extends to everyone and everything: he's equally comfortable dissing the BBC's Nationwide (remember that?) and venting a bile-filled but fundamentally quite accurate deconstruction of the appeal of punk rock. The juxtaposition with his scripted castle commentary is a witty delight, and from time to time we dip into Grundy's memories to recollect his life before his fall. The notorious Sex Pistols meeting itself is re-enacted under a cold, nightmarish light.

The pace sags a little in the middle, the waspish one-liners fading away as the script becomes tied up in wordiness. On one level this is completely intentional – Grundy loses his punch and sharpness as the frequent swigs from his hip-flask kick in – but I did find I missed the sheer laugh-aloud entertainment which defined those opening scenes. In its place, however, we get something more thoughtful: it's here that Ankur Sengupta comes to the fore as floor-manager Julian, puncturing Grundy's self-importance with hard questions about what his bitterness truly reveals.

But Julian's on to a loser with that one. Grundy embodies so much we ought to despise – his arrogance, his intellectual snobbery, the prejudice he expresses in language if not in deed – and his self-evident reliance on alcohol might invite our pity, too. Yet there's something about his rakishness we can't help respecting, a magnificence to his truculent constancy in the face of a changing world. It's in maintaining that impossible balance – keeping those two repelling magnetic poles somehow aligned – that Tim Connery's rumbustious script most triumphantly succeeds. This is an enlightening and engaging insight into Grundy, not only well-written and funny, but also offering a possible insight into how that infamous interview ever turned out the way it did.