The culture of secrecy

As one might expect, the UK's secret services are, well, secretive.

The announcement that Dame Stella Rimington, head of MI5 from 1992 to 1996, has written a "personal" memoir has shocked many.

Professor Ian Leigh, from Durham University's law department, has written extensively on the role of the security services.

"I don't understand how you can chase people like David Shayler for talking about their work at MI5, then allow a former head of the service to publish a memoir."

Dame Stella has finished a 'personal' memoir

Mr Leigh says the legal principle for any retired agent tempted to recount their past is quite clearly laid out in the Official Secrets Act.

"There is an absolute prohibition against former members of the security service revealing anything about their work."

The government has said a draft copy of Dame Stella's book will be checked before publication to ensure national security is not compromised.

Mr Leigh says such a move represents "partiality" on the part of the authorities.

Open secret

Of the security services, MI5 is perhaps the one ordinary citizens know most about, in part thanks to the loose lips of former agents, such as David Shayler.

Charged with addressing threats to national security, MI5 is the section of the security service agency also mostly likely to affect the daily lives of British citizens.

David Shayler talks openly about MI5

Under Stella Rimington and her successor Stephen Lander, MI5 has marketed itself as the cuddliest of the secret services, hoping to dispel its "secret police" image.

The agency openly placed job ads in the papers, compiled recruitment packs for graduates, launched a website and set about exploding the popular "myths and misunderstandings" fostered by its decades in the shadows.

Speaking up

The service does not have its own press office or bandy around its phone number. But since 1998, it has encouraged those with information to get on the blower, with the unveiling of a special hotline.

MI5 claims not to be fanatical about secrecy.

It has boldly announced that the staff canteen serves a mean chicken Madras and that the carpets in its London HQ are blue.

Can secret agents be more cuddly?

How much the carpets cost to fit is a slightly more touchy subject at Thames House, however.

MI5 and its sister service MI6 have been resisting the disclosure of a National Audit Office report into the refurbishment of their swish offices.

Mr Leigh says the agency seems more willing to release certain gems of key information.

"In some respects it is less secretive now. We know how many files it has, or more exactly how many people it has files on. We also know how many people it employs."

Facing the public

Mr Leigh says today's MI5 also has a "public face". Departing from the low-key approach of her predecessors, Dame Stella even delivered the BBC Richard Dimbleby Lecture while in charge.

The Commons' Intelligence and Security Committee, under MP Tom King, has also become a "semi-official route" for information on the work of all the security services.

Mata Hari: MI5 revealed the early years

To its credit, MI5 has given access to records dating from its early years. But the Boys' Own adventures of founders Vernon Knell, "K", Mansfield Cumming, "C", and their band of Boy Scout messengers in 1909 shed little light on the modern service.

"If you talked to people in the service, they'd argue there were good reasons not to divulge past actions," says Mr Leigh.

Though MI5 may be keen to show it can keep faith with secretive former informants and operatives, the case for presumably outdated "tradecraft" staying under wraps seems less convincing.

Paper chase

One historian told BBC News Online he abandoned his study of the service, having been chastised for even admitting to a journalist he had met MI5 officials to discuss access to documents.

Other critics have suggested MI5 is keener to gain publicity than lay itself open to investigation.

For many of the juicer details, journalists have to rely on files released by other countries, such as the MI5 dossier on John Lennon given to the FBI, or on the revelations of former agents themselves.

Wright stuff: Ex-agent alleged a plot against Wilson

In 1987, retired operative Peter Wright wrote Spycatcher. It alleged MI5 tried to discredit Labour prime minister Harold Wilson, something the service still denies vigorously.

David Shayler, a fierce critic of the MI5 "bureaucracy", has floated a number of tales about the agency's shortcomings from his exile home in Paris.

Mr Shayler, who joined MI5 in 1991 after spotting the cryptic job advert "Godot isn't coming", has described his passage through the organisation.

He worked for G Branch, dealing with international terrorism, C Branch, where government officials' backgrounds are checked and T Branch - targeting terrorism in Northern Ireland.

However, no matter how limited MI5's own attempts to open up, compared to the espionage service MI6, based further up the Thames in Vauxhall, the service is positively verbose.