Splendid! Splendid! The Authorised Biography of Willie Whitelaw, by Mark Garnett and Ian Aitken

John Campbell enjoys an affectionate biography of Mrs Thatcher's trusty right-hand man

When Margaret Thatcher delivered her immortal observation that "Every Prime Minister needs a Willie," she meant two rather different things – apart from the gloriously ironic double entendre, of which she was initially unaware. First, every PM needs an authoritative deputy to chair committees, resolve disputes and ward off trouble. But she also meant that a PM needs one senior colleague with no ambition of his own to guard his (or her) back against the plots of jealous rivals.

The two roles overlap, but neither John Prescott under Blair nor Michael Heseltine under Major has come near filling both. Perhaps the closest approach was Ernie Bevin under Attlee, who dealt brutally with the periodic challenges of Herbert Morrison and Stafford Cripps.

Whitelaw's value lay in the fact that he had contested the Tory leadership with her in 1975 and been soundly beaten. He came from the opposite wing of the party and represented everything she wanted to overthrow. He loathed her hard-faced monetarism as much as she despised his soggy liberalism.

Yet, having lost, he conceived it his duty, in an almost military sense, to serve her as a loyal deputy and give no countenance to his old friends. He became her hostage, whose suppression of his own views made it impossible for Pym, Prior, Gilmour and the rest to mount any challenge to policies which they – and he, privately – feared would be disastrous.

The roots of Whitelaw's self-denial lay in his background – Scottish landowning with a tradition of public service – and experience in the Scots Guards, where he found his level as second-in-command of his tank battalion. He made his mark politically as Ted Heath's Chief Whip in opposition and then as his right hand in government. He was already seen as one of nature's deputies. But their relationship was disrupted in 1972 when Heath appointed Whitelaw Secretary of State for Northern Ireland, when Stormont was suspended.

His combination of what Mrs Thatcher later called "bonhomie and steel" made him the perfect choice. Despite serious errors of judgement towards the IRA, Whitelaw achieved in 18 months the framework of a power-sharing agreement which did not stick in 1974 but which remains the basis of all progress made in the past quarter-century. This biography rightly gives more space to Whitelaw's time in Ulster than to anything else.

Unfortunately, Heath brought him back from Ulster at the critical moment to try to avert a miners' strike. But the unions saw him as a soft touch and were not appeased. The result was the three-day week, the loss of the February 1974 election and, eventually, Mrs Thatcher as Tory leader. This was a ghastly time for Whitelaw. He felt obliged to stand against Thatcher, though his incapacity for leadership was already clear. From this trauma grew the incongruous partnership of the next 13 years.

Whitelaw had a torrid time as Home Secretary in the first Thatcher government, assailed by everything from inner-city riots to an intruder in the Queen's bedroom. He found his role after 1983 as Leader of the House of Lords, where his reputation as a consummate operator – behind a buffoonish exterior – went from strength to strength.

In fact, it is hard to be sure precisely what he achieved. The government still suffered a succession of self-inflicted disasters, from Westland to the poll tax, of exactly the type he was supposed to prevent. It must be assumed that he did prevent many more; certainly, he exercised a restraining influence in some areas – notably Mrs Thatcher's animus towards the BBC. But the best evidence of his effectiveness is that the government began to fall apart after he suffered a minor stroke and resigned in January 1988.

Whitelaw remains an elusive subject, remarkable less for what he did than for what he was. Mark Garnett and Ian Aitken – an effective combination of historian and journalist – have made a good job of this frankly affectionate biography. The only regret is that access to Lord Whitelaw's papers has yielded so few revelations. The sad fact is that politicians, even of Whitelaw's generation, no longer write letters.

The second volume of John Campbell's biography of Margaret Thatcher will appear next year