At the moment when the announcement was made, in 1976, I was standing to attention on the runway of Sofia’s airport, clutching a sheaf of giant gladioli and listening to the eighth or ninth verse of Bulgaria’s national anthem. Eddie Bolland, the British ambassador, whispered to me through the foliage: “The prime minister has resigned.” I whispered back: “Was he the big noise or is that the Communist party first secretary?” Bolland abandoned diplomatic discretion and shouted his reply. Not their prime minister. “Our prime minister.”

I was not alone in my astonishment. Harold Wilson had told his usual confidants that he would leave Downing Street during his 60th summer. But, unusually, they had respected his confidence. So newspapers were left to speculate about the “sudden decision”. All the old canards were dredged up and regurgitated to produce a consensus that he had something to hide. Wilson took his place in British history as the prime minister who deserted his friends and abandoned his principles.

Nick Thomas-Symonds, a Labour MP (and shadow secretary of state for international trade), has challenged the caricature in a meticulously researched and elegantly written biography. Indeed, as the title makes plain, Harold Wilson: The Winner seeks to do more than correct the record. It celebrates the life and work of a politician who “made Britain a fairer place”, was “one of the 20th century’s great personalities” and whose perceived shortcomings were trivial compared with his achievements.

Wilson led his party to victory in four general elections. That, itself, goes some way to justifying Thomas-Symonds’s claim. But much of what might be described as achievements were the result of “preventive action”. Wilson avoided civil wars in Central Africa and Northern Ireland and steadfastly resisted American pressure to send British troops to Vietnam. They are not the sort of successes that attract a place in history. Preventing, or at least postponing, a Labour party split is probably more important to Thomas-Symonds and me than it is to posterity.

A prime minister is invariably held responsible for catastrophes that – in the fashionable phrase – occur “on his or her watch”. But they rarely receive credit for the successes of their years in office. Wilson’s first administration was one of the great reforming governments in British history. Without the prime minister’s blessing, parliamentary time would never have been found to abolish capital punishment, liberalise the laws on homosexuality, divorce and abortion, or for the first positive action to promote racial equality. We should think of Wilson as the architect of social reform.

Admittedly, Wilson is held almost solely responsible for the decisions that prejudiced the prospects of his first government and hastened its ignominious end. There is no doubt that he personally vetoed devaluing the pound in the autumn of 1964. And five years later, even Barbara Castle – his friend and constant champion – accused him of “betrayal”, because he would not support her plans for industrial relations reform. But a flexible interpretation of policies and promises has never excluded a prime minister from the pantheon of great politicians.

Abandoning old policies can be, and often is, forgiven. Abandoning old friends is not, and the real doubts about Wilson’s instinct for loyalty began when he edged away from Aneurin Bevan, one of the authentic heroes of the Labour movement. Bevan resigned from the Attlee government in protest at what he called the “imposition” of health charges. Wilson resigned shortly afterwards. But he chose to point out that he was opposed to the whole drift of the government’s economic policy, “not just the levy on teeth and spectacles”. It was assumed that he made the distinction in the hope of trivialising Bevan’s rebellion and capturing the leadership of the Labour left.

Paradoxically, damage to his reputation came about because of his undying loyalty to his private and personal secretary. The index of The Winner lists 68 references to Marcia Williams, Lady Falkender, twice as many as any cabinet minister. I do not know or wish to discover the nature of the Wilson-Williams relationship, though if pressed I would guess that it was not what the prurient press hoped it to be. Yet for some reason she was allowed to behave in a way that did Wilson great damage. The final blow was the “lavender list” – Wilson’s nominees for dissolution honours. It was written on Lady Falkender’s notepaper, and included names of men Wilson barely knew. Somehow it found its way into the newspapers.

I know beyond doubt that Williams fed Wilson stories about rightwing dissidents in the parliamentary Labour party plotting his downfall. Although much of the scheming was imaginary, the antagonism was real. It could be traced to his decision in 1960 to challenge Hugh Gaitskell for the Labour leadership at the time when Gaitskell had promised to “fight and fight again to save the party we love” from the suicide of extremism. He lost, but in 1963, after Gaitskell’s death, Wilson took the leadership, and a year later led Labour to victory.

Old Gaitskellites served in his government but felt no obligation to hide the disdain they felt for a man they regarded as a usurper. Wilson had committed the unforgivable sin of not being Hugh Gaitskell. Thomas-Symonds, free of such prejudices, leaves the reader in no doubt that Harold Wilson was a good prime minister – but hardly a great one.

• Roy Hattersley was minister of defence and minister of state for foreign affairs in Harold Wilson’s government

• Harold Wilson: The Winner by Nick Thomas-Symonds is published by Weidenfeld & Nicolson (£25). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply