Even in retirement accomplished spy-catchers give little away.

Either to stay on the right side of any pension arrangements, or continue to keep loved ones in the dark, they reserve their truly scandalous secrets for the grave, leaving post-burial historians to do the digging, and thriller writers to put fictional flesh on the bones.

Tomorrow, though, might be different when Eliza Manningham-Buller delivers the first of three Reith lectures for Radio 4. Ever since 2007 when she stood down as director general of MI5, she hasn’t shirked from saying out loud what governments don’t wish to hear. Last year her testimony to the Chilcot inquiry into the lead up to war in Iraq amounted to a scathing indictment of the Blair administration.

Ms Manningham-Buller made it clear she warned the Prime Minister that invasion -- on the basis of “fragmentary intelligence” -- would “undoubtedly” increase the terrorist threat to Britain. But her advice was superseded by that from MI6 which, she said witheringly, had “over-promised and under-delivered” when it came to Iraq. Rivalries between national security agencies are nothing new, of course. Those trained in covert activity never stop being watchful of everyone.

In the early 1980s the British traitor Michael Bettaney, working in MI5’s counter-espionage unit, shared an office with two of Ms Manningham-Buller’s assistants. At the time she was one of only five people who knew that Oleg Gordievsky, deputy head of the KGB at the Soviet Embassy in London, was actually a double agent. Mr Gordievsky’s evidence was crucial to Bettaney’s Old Bailey conviction in 1984, but had his name been mentioned previously in Bettaney’s presence, the Russian would have been the target of an inevitable Soviet hit squad. Later, Mr Gordievesky acknowledged that Ms Manningham-Buller’s ability to keep a secret had saved his life.

Recently the political philosopher John Gray downplayed the significance of the British “ice-cold deceiver”, Kim Philby, because, in gazing outwards from his Moscow flat, Philby failed to predict the future. Today cyber-intrusion deludes us into thinking there are no secrets left to leak; even WikiLeaks -- which last year “revealed” much already guessed -- was overtaken by the more immediately gripping sensations of News International’s hackgate.

So, how do spies now occupy their time? We’re beyond the days when spooks looked dingier than a private eye’s raincoat, walked with a limp and had half a finger missing. In Russia itself some veterans, who acquired a gourmand’s girth while sleuthing, have become food writers, recalling past moments in grand restaurants beyond the Iron Curtain, when, seduced by sensual vapours from the kitchen, they put the Motherland on hold and forgot to activate the tape recorder hidden in the table napkin.

My only brush with a spy began at the Soda Fountain in Fortnum and Mason when Misha -- not his real name -- was the appointed Russian official organising my trip to Moscow to report on Mikhail Gorbachev’s reforms. His wife, he grumbled, hadn’t taken to London. So she stayed indoors bringing resentment to the boil. And the media didn’t help matters, he said. “I take you journalists to lunch. I buy you alcohol and lobster thermidor, and in return you tell me nothing.” That should have been a clue, although the Moscow assignment proceeded without any creepy incidents: no bugged cocktail olives, no lumpen stalkers watching morosely from the shadows.

But six months later Misha was among a dozen Soviets evicted from Britain for spying. On television that night we caught a glimpse of him at Heathrow, looking like an unhappy whippet. In tomorrow’s recorded lecture, MI5’s former boss will refer to 9/11 by exploring whether that heinous event was a terrorist attack or an act of war. “Or was it something different?” Dame Eliza Manningham-Buller may be a retired spy-catcher, but she still knows how to raise an alert by asking a cryptic question. 

The Reith Lectures begin on Radio 4 on Tuesday at 9am.