"There are several collections here that are closed and not on any list: we cannot even declare that we have them," says Alan Kucia, senior archivist at Churchill College, Cambridge.

Built in 1974 to house the 3,000 boxes of documents and letters that Sir Winston had accumulated, the Churchill Archive now contains the papers of 460 politicians, scientists, civil servants, diplomats, soldiers, sailors and airmen. It includes the papers of Clement Atlee, Lord Balfour, Ernest Bevin, Fenner Brockway, Neil Kinnock and - running her hero close with 2,500 boxes - Lady Thatcher. Many of these politicians' papers are closed because they name living individuals - in particular constituents - whose rights under the Data Protection Act would be infringed if details of their health, sex lives or political affiliations were made public.

Others are closed because they are official documents: in this case the archive has to contact the relevant government department and ask if papers may be opened once the 30-year public records deadline has expired. Any document relating to the honours system has to be closed until the recipient of the honour has died.

Undoubtedly there are big secrets: among the Churchill papers, for example, are 12 closed boxes. The archive is not even allowed to say what these boxes contain, but several are known to be closed on the orders of the royal household; others deal with undercover intelligence in Ireland in the 1920s and code-breaking research at Bletchley Park.

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Some of the archive's holdings are not available simply because they have not been catalogued yet: into this category fall the papers of Lord Hailsham and Selwyn Lloyd. Work on Thatcher's papers began recently: initially her office said they would be closed to everyone except her official biographer, says Andrew Riley, the archivist who is working on them. Now it appears that access may be more generous - but sorting the papers will take at least three more years.

The college tends to be offered collections because its name is known, Kucia says. "When Kinnock resigned after the 1992 election, his chief of staff rang us and said 'Would you be interested in having the papers?' We said yes, and a few days later the pantechnicon arrived."

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The Kinnock papers are already open to researchers who receive the former Labour leader's permission.

And although colleges will agree to keep papers secret, even to the extent of hiding their very existence, Kucia says: "Generally as a collection we don't want to take things with a long closure unless there is good reason. There is limited storage space, and we want to take stuff where there is an expectation that people can see it."