In The Secret Art of Government, to be screened on BBC 2 on 13 June, Lady Thatcher returns to Number 10 specially for a programme which reveals for the first time the artistic taste of the Establishment.
Since the election, BBC cameras have been filming in Downing Street, ministerial offices, the British embassies in Paris and Cairo and the GAC's private headquarters in Soho.
In the last year the new paintings installed have taken on a key role as part of New Labour's Cool Britannia policy.
Lady Thatcher makes no attempt to conceal her dislike of the most radical shake-up of the collection since it was founded in 1898, which has swept aside the more traditional tastes of Tory inhabitants. Her reaction is, unsurprisingly, utter dismay.
Lady Thatcher has always displayed least admiration for modern artists and once said of sculptor Henry Moore: "I don't like the big lumpy ones."
The former prime minister tells the programme: "I think if you have what I call the geometric ones - just pure different squares, triangles and heaven knows what put together - I don't think they go with the general background.
"I must tell you some modern things are terrible - I can't imagine people wasting money on them. But other people's tastes are different and what I think looks really rather like a horrid big spider crawling around might in fact be a work of genius in a couple of centuries."
Present and past government ministers - from Chris Smith and Mark Fisher to Peter Lilley - are among the contributors to the programme.
Chris Orr, the artist of a series of erotic etchings, has expressed delight that his work has been hung in Peter Mandelson's Whitehall office. Mr Orr said he knew the drawings, done in the Seventies, had been bought for the collection but did not know where they had been hung.
"I'm extremely pleased that Peter Mandelson appreciates any kind of narrative art," he says.
John Major, who is also interviewed, also found the new regime has showed a healthy disdain for his taste. The day after the election, out went the Badminton Game, by David Inshaw.