My next book - a history of Eritrea to be published in December - is in its final stages. As I trim bloated chapters and check references, a gleeful realisation dawns. With any luck, I may never have to use the Public Record Office in Kew again.

Libraries have always spooked me. It started in my undergraduate days, when within minutes of laying out my files in the college library, a terrible sense of futility would descend. For those who suffer from this form of allergy, no venue can be more intimidating than the Public Record Office, where 100 miles of governmental records, released under the 30-year rule, lie waiting.

On entering, your belongings are frisked with a thoroughness a Belmarsh warder would appreciate. The hush inside is more than mere silence: the air-conditioning has been set at a level deemed best for preserving paperwork. On the lookout for thieves, television cameras are trained on the desks. Patrolling guards lean over to check that you are making notes in pencil and scold you if you lean too heavily on a book.

It's a place of pilgrimage for researchers from across the world, Malaysians and South Africans, Egyptians and Indians, Canadians and Sudanese. Bent on understanding the workings of the British empire, they buy their tickets to Kew Gardens, trot down a suburban street and enter the library - built, disconcertingly, under the final section of the Heathrow flight path.

They love it for its quiet efficiency. Records are computerised. Once you have learned to operate the system, you can whip through files with remarkable speed. But above all, they love this library because they feel, with every "secret and confidential" file they open, that they are ripping off the smooth mask of British officialdom to see the cogs of decision-making churning below.

It's a thrill even I have experienced. If you have been subjected all your life to the official bland account, an ambassador's devastatingly frank memo back to London carries an electric charge. Gasp at the racist language of the first secretary in Nairobi! Blanch at the sexism of a Foreign Office minion! Shiver at the cynicism of the minister's Question Time performance! At its best, a day at Kew can be a personal Hutton inquiry. History's very essence, you feel, is captured in this paperwork.

Or rather, I used to feel that. Because, on my last visit, I had an epiphany. Reading through some reports sent to the Foreign Office by diplomats based in Ethiopia in the 1970s, I came across two howlers. These glaring mistakes should have been obvious to anyone based in Addis Ababa at the time.

It set me thinking. During my weeks in Kew, I had been reading memos written by exactly the kinds of people I routinely interview as a roving foreign correspondent: ambassadors, first secretaries, political counsellors. Only rarely have I emerged from an embassy thinking: "What a useful briefing." Usually, one exits fuming, having spent an hour updating the official concerned on conditions in his own backyard.

Some diplomats are wonderful sources of knowledge; many are hopeless; most no better-informed than a hack who does a modicum of homework. I know their faults, because foreign correspondents share them: they don't get out enough, they socialise too much with their own kind, they don't know the local language. The longer I have been a journalist, the less value I have come to place on diplomats' opinions. So why was I treating the reports drafted by their 1970s equivalents as the source of all truth?

For many historians, there is an obvious answer: because this is all that remains. But in my case, this wasn't true. I had been stunned to discover how many of those who lived through Italy's 1935 invasion of Ethiopia, for example, were still alive; likewise those who worked for Haile Selassie in his prime. Frail, in their trembling nineties, but still compos mentis. Yet when one of these eyewitnesses told me something unexpected, I made a mental note to check it against the written record; when I came across a suspect claim in a memo, I accepted it unquestioningly.

The answer, I think, lies in the spell cast by the written word. It is extraordinary what power paper and ink hold. The swear word that passes unnoticed in speech seems shocking in an e-mail. Typed up by the market researcher, back-of-the-taxi truisms become a penetrating analysis of contemporary thought. Add the seductive word "secret" to the mix and a statement of the bleeding obvious is transformed into a priceless insight. "Just remember that it always looks better in print," is one of journalism's most important, if least acknowledged, maxims.

That day in Kew, I realised that paperwork is as fallible as the men who produce it - which is very fallible indeed. In future, I will be spending a lot less time in Kew, however amazing its facilities, and a lot more listening to the veterans. And I urge my Egyptian, Sudanese and Indian colleagues to do the same.