‘Elle est fou. Ma mère est loony. Ma mère est complètement . . . weird.” Thus speaks Winnie, the eight-year-old heroine of Alan Ayckbourn’s 73rd full-length play My Wonderful Day who is brilliantly and most convincingly portrayed – in a performance that is nothing less than a tour de force – by an actor (Ayesha Antoine) who is fully 20 years older.

But why the French? That is actually one of the weirdnesses, as some might perceive it, of Winnie’s mum Laverne, a garrulous domestic cleaner endearingly presented by Petra Letang. A generation or two back, her family emigrated from Martinique. Now, abandoned by her husband, she dreams of going to live there with her daughter and soon-to-be-born son. In the meantime, so they are ready for the language of the island, every Tuesday is a French-only day And today is a Tuesday.

It is also going to be the ‘wonderful day’ that Winnie has been asked by her teacher to describe. She is skiving off school, after a sniffle or two has conveniently suggested a possible cold, and has accompanied her heavily pregnant mum to her work in the lavishly appointed home of TV personality Kevin (the ever-reliable Terence Booth) and film director wife Paula (Alexandra Mathie).

Pen and notebook in hand, Winnie is soon recording all that is happening around her. And what events they are! It soon becomes apparent that the glittering couple are non-speaks, she having fled the house after food-throwing ructions the night before provoked by Kevin’s infidelity with work colleague Tiffany. Barely has this news sunk in than the upper-crust airhead ‘Tiffy’ (Ruth Gibson) herself arrives, soon followed by another of Kevin’s business associates, the badly hung-over Josh (Paul Kemp). His personal sadnesses point up the theme of domestic disharmony that runs through this funny but – as is usual with Ayckbourn – touchingly true story.

The imminent arrival of Winnie’s baby brother ‘Jericho’ (!), announced by his mother’s waters breaking, necessitates Laverne’s sudden departure for the maternity hospital. Left behind with her new protectors, Winnie observes with wide-eyed wonder – but also with what seems to be a precocious understanding – the increasingly adult activities taking place around her. These culminate in the arrival – in true farce tradition – of the dragon-like Paula (Alexandra Mathie).

Occasionally, Winnie is involved herself in the action, as when she hilariously puts Josh to sleep with her halting reading of chapter four of Frances Hodgson Burnett’s The Secret Garden (whose upstairs/ downstairs theme reflects other ideas in the play). Only a playwright of the experience – and genius – of Ayckbourn could see the comic potential in that.

As the director as well as the writer Ayckbourn is able to work wonders on the stage. He knows what he wants – what we want – and he knows how to encourage it from the performers. Ayesha Antoine writes in a programme note of “everybody working so hard to make Alan’s precise vision come alive”. Anyone seeing this witty and uplifting play this week will recognise how completely successful their efforts have been.

Oxford Playhouse, until Saturday. Tel: 01865 305305 (www.oxfordplayhouse.com).