It's that time of year again when I don't even mind going to the theatre on a Sunday, especially if the writer is Alan Bennett. The double bill of familiar recollections from his diaries and autobiography, Hymn and Cocktail Sticks, was a real treat yesterday afternoon, part of a perfect pre-Christmas weekend.

Good weather, neighbours' parties, a birthday brunch: my son shares a birth date with Beethoven, Noel Coward and Christopher Biggins, a coincidence always worth celebrating, I feel, as a guarantee of musical profundity, levity and cheeky vulgarity all in one, something like Jan Kodaly's Hary Janos suite, perhaps. Eggs benedict, games with my grand-daughter and half an hour in the local park in Kilburn set me up nicely for the South Bank outing.

And what a delight it is to push and shove your way through the winter market stalls along the stretch of walkway between Waterloo Bridge and the National Theatre: wooden toys, colourful jewellery, ostrich and kangaroo burgers (they'll put a spring in your step), a huge tureen of spitting bratwurst, Italian liquorice in all flavours (vanilla and water melon my favourites), Mexican tacos and churritos.

And the National itself, crammed in the foyers, full in the Lyttelton stalls for Alex Jennings' brilliant physical impersonation of the Yorkshire sourpuss, most of the audience giving an impression of having a good time between last-minute shopping and a nice Sunday dinner to come.

I was home in time to sit at my desk for a couple of hours then go a few doors up the road to sing carols and sip mulled wine with my neighbours, Simon Chandler and Nicki van Gelder, and their guests who included our mutual neighbour Gemma Jones (about to start rehearsing with three different sets of children in The Turn of the Screw at the Almeida), Jemma Redgrave, comedian Simon Amstell (who comes from Jewish Ilford, or Gants Hill as it's known), Victoria Hamilton and her husband Mark Bazeley, and many more.

We'd all bought finger food and a bottle, while Simon stirred a huge pan of mulled wine that looked more dangerous than the witches' brew in Macbeth, though not perhaps with fillet of a fenny snake, or eye of newt and toe of frog.

Hamilton and Bazeley now have two young boys, aged just one and three, so I sympathised with her on how exhausting it must have been to play in Mike Bartlett's tripartite epic Love Love Love at the Royal Court earlier this year. Hamilton gave one of the greatest performances ever seen, even on that hallowed stage, as a woman ageing across three decades, from dippy hippiedom to rancid middle-age.

Although the performance was, I think, noted in the Evening Standard "long list," it seemes to have missed out on the awards nominations it really deserved. Maybe the Oliviers and the Critics Circle will redress the balance. Another performance I particularly admired this year was Pip Carter's in Nick Dear's Dark Earth, Light Sky at the Almeida, and I don't think anyone's going to give him an award, though he certainly gets my vote.

Carter as the poet Edward Thomas conveyed a sense of what it is to be a writer, and to find your voice, despite all manner of personal misgivings, more eloquently and tactfully than anyone else I can remember. Understated acting is perhaps an oxymoron, on the stage at least, but this was a rare example of just that, and deeply affecting.

As a personality and a writer, Alan Bennett is a world away from Thomas, and yet Alex Jennings still manages to catch that wonderful reflective process in Bennett which transforms simple, modest observation into flashing, rapier sharp critical wit. And for such a shy, retiring person, Jennings suggests there's a flamboyant show-off bursting to get out, in his own way. We know nothing about Shakespeare, but everything (and more, it sometimes seems) about the confessional Camden Town bicyclist with the floppy blond fringe.

And what's this, a show-off in the critical ranks, too? Michael Billington took his grandson to see Jack and the Beanstalk at the Theatre Royal, Stratford East, and reports in today's Guardian that the little lad was invited onto the stage: "Although the cast is first-rate," says Billybong, " I couldn't help feeling they were given a run for their money by my grandson, who acquitted himself with alarming aplomb."

Actually, "alarming aplomb" is the right phrase for Alex Jennings, too, whose physical portrait of Alan Bennett is a masterpiece of joyous, affectionate impersonation that crosses a boundary into seriously good acting.