The 30th anniversary of Margaret Thatcher being given the keys to No 10 Downing Street is a birthday some wish to celebrate enthusiastically. Others may prefer to celebrate quietly at home. And, among her opponents, there are still many who bitterly regret that she was ever Prime Minister. Yet all these groups agree on one thing — on May 4, 1979, Britain crossed the political threshold into modern history.

Many of the arguments that dominated the political debate before Mrs Thatcher came to office now seem extraordinary. Did the Government really try to control inflation by setting the price of supermarket goods by committee? Did it really prevent citizens from spending currency abroad? Did it really try to settle national strikes by according union leaders semi-official status? That no mainstream politician would dream of advocating any of these things now is testament to her political success.

It is hard to recall that when Margaret Thatcher set her face against prices and incomes policy, union power and exchange controls it was she who was considered outlandish. It was regarded as inevitable that her policy of driving down inflation without an incomes policy would have to be reversed when unemployment became unmanageable. She would be forced to make a U-turn. And perhaps, had the Falklands conflict and the choice of Michael Foot as Labour leader not intervened, she would indeed have been forced to choose between a U-turn and political defeat. She would now be remembered as a political cautionary tale rather than as a political heroine.

Fortune, however, favoured the bold. Mrs Thatcher was able to see through her programme while enjoying fantastic political success. Even now the policies she pursued are hotly controversial. So is her extremely combative personality. But her answer to the critics is simple — what she did was necessary and overcoming resistance to it was hard. The social dislocation experienced by some was, however regrettable, hard to avoid. The pain felt during the medical procedure she undertook resulted from the extent of the injuries she was seeking to heal rather than from the callousness of the surgeon or the refusal to take an alternative course.

There are criticisms of Margaret Thatcher’s governments that stand more scrutiny than those that suggest a softer landing was possible for the British economy. It is unfortunate that she did not accompany her economic liberalism with more political and social liberalism. And the tone of her administration sometimes moved from being necessarily tough to being unnecessarily arrogant. There is also a criticism from the Right — that she did not do enough to reshape public services or reduce the size of the State.

Yet against these criticisms is another simple fact. It was not simply economically that the Thatcher governments achieved a transformation. Socially they challenged the elitism of closed institutions and the pessimism of the Establishment. Margaret Thatcher stood for modernisation, meritocracy and optimism about Britain’s future.

To David Cameron now falls the job of making sense of the Thatcher legacy. Once he may have been seen as the “heir to Blair”. But now, as he prepares to run as a deficit cutter and govern as a deficit cutter, he will realise that he is very much the “heir to Thatcher”. He will face the difficult task of showing her boldness without being seen as similarly combative, of showing her toughness without being thought as harsh as she was, of retaining popularity without being able to rely on Michael Foot for assistance.

The experience of Margaret Thatcher tells him how difficult things will be. But it tells him something else — success is possible.