In the Mackay canon all characters transform before your eyes: boy becomes girl becomes mugger; respectable woman shoplifts babies. It is perfectly suitable that the purveyor of such tales, the enchantress whose language sparkles even as it delivers the punch, should be this gentle, innocuous 48-year-old woman sitting demurely over tea at the Ritz, slightly discomfited by the low-slung chairs.
Back in the Sixties when her first novel was published - she was 20 - Mackay was an overnight literary sensation, a girl wonder splashed across Sunday supplements. She had a play on at the National, the film rights to her book sought by Albert Finney and her views canvassed by Marge Proops. Then came marriage, and three daughters in quick succession.
The received view is that Mackay languished in suburbia, early promise unfulfilled. Still, she has written, over the years, seven novels and two collections of short stories. Each time there has been a burbling of excitement, great reviews, occasional awards. 'I was leading a very unliterary life while the children were growing up. Well, it's impossible to lead a literary life with children if you live in the country,' she says. 'Then I won a prize in a Radio 3 competition, and that did it.'
Her ordinary life has been marked by ordinary horrors. Her earliest memory is of falling into the pond at Hampstead Heath. Her childhood was very happy some of the time, but her parents were not happy and her mother would periodically leave, with the children in tow. They finally divorced after Shena left home.
She was happy at Tonbridge Grammar School, but miserable when she was moved to Kidbrooke Comprehensive, which was a prototype and showplace. 'I just felt ugly all the time there,' says Mackay, who is beautiful, in fact. 'I didn't have the proper uniform and they made my life hell. My uniform was on order from the Co-op, and it kept not coming. I used to go and it wasn't in, and it wasn't in . . .'
She regrets not going to university, though at the time she couldn't leave school fast enough. 'But I realised later in life how you miss out on the whole friendship thing; if you go to university you can have a whole block of friends, some of them for life.'
Mackay's mother died last year after a long illness. Shena visited regularly, travelling up three times a week, from Norwood, where she lives, to Woodside Park. 'Perhaps because she was ill, my sister and I saw more of her than people of our age normally do see of their mothers. Several times she wanted to die. So, it brings some relief, but it is a very great change in my life. Huge blocks of time, as well, that one used to spend visiting.'
Divorced now, and her daughters grown, Mackay is famous once more. She is hero-worshipped by Julie Burchill in Elle magazine, and later this year there will be a Bookmark programme in which she plays herself in a drama. Burchill is giving a party to launch her new book. 'There have been some snide remarks because of Julie,' says Mackay, 'but the thing about Julie is, she does care, she cares passionately about writing and good books.'
Mackay's latest book is a collection of short stories, The Laughing Academy (Heinemann, pounds 13.99). 'The characters are always very clear in my head. Their hair colour, and eyes and so on, although I don't always describe them fully, I realise. I usually get a vague picture or outline first - well, sometimes it's a very vivid picture as with Olive and William in Dunedin (her last novel) - others I know what they're like but I have to concentrate on my thoughts to put them on paper.
'I always know how a story is going to end, and I very often have the last line first, but some of it just grows organically as I go along. I suppose I have the plot unconsciously. I find it hard to start though, unless I've got a title - it's a silly sort of thing, but that makes it easier.
'I work fairly slowly because I try to get it right before putting it on paper. I do a lot of work when I'm asleep. If you just catch it before you wake up - you retrieve some fantastic puns that way,' she grins.
She is now at work on her next novel, having sworn never to write another long book after Dunedin. 'I just feel I can now - I no longer feel so mentally exhausted.'
For me, her short stories work better than any of her novels. The polemics in Dunedin compromised the plot, whereas in the stories she seems stricter about not letting her - very active - social conscience interfere. But publishers prefer writers who can go the full length, which seems short- sighted in the soundbite age. 'Short stories do count for me, and for a lot of people,' says Mackay.
'I'm quite happy now. I've been through times of great discontent when I've felt . . . well, mainly the money thing - if you're worried about money it just cripples you, you can't concentrate - but it's no longer the problem it was.' Reviewers always say she has the alchemist's gift. Any page of her books bears witness to that. Even a white telephone hanging on a kitchen wall becomes blancmange that has been hurled there. In The Laughing Academy she writes: 'The autumn sunshine, gilding brick and berried trees, was beneficent, like a matriarch bestowing gold and jewels on her heirs, all their sins forgiven; the plane trees were dappled benign giraffes.' In better educated times, we would be giving our children passages of Mackay to learn by heart, as gifts for later life.