For sale: des res for the owner with an eye on security

The occupants have gone but Mary Braid scents secrecy in the corridors that once housed MI5

To property developers the nine-storey building covering 3,930 square metres in the heart of Mayfair, central London, is the freehold opportunity of the decade.

But to those who wander along the miles of narrow, dark corridors leading to hundreds of shabby, stripped offices at 1 Curzon Street there are still a few clues in the echoing shell to suggest this is a little more than a piece of prime real estate.

Even for the security conscious the thick metal bars on just three windows in the corner of the sixth floor seem a little excessive. The imprint of a former partition on the worn carpet implies that the windows belonged to a separate room, a veritable stronghold.

The fire and bomb instructions still stuck to the wall are a touch unusual. On the sounding of an alert - red, amber, black special and black - staff are instructed to first "put away" documents and then "pick up personal possessions" before heading for safe areas. Then there is the transparent casing around all the wires - even the light switches are see- through.The feature seems more anti-bugging than wacky interior design. The sign in the basement beside the double steel security shutters warns "Suspected Hostile Activity, Consider Counter Measures".

If you had not guessed that the last group of MI5 staff only recently left the building there is always the button on the first floor which, with a loud groan, lowers steel shutters on all the windows. Along the corridor is a reinforced concrete, windowless "bomb safe room" which might accommodate 200 people - not all the staff - beside a double-height filing hall protected by a thick iron door and prison-style bars. Like all the other rooms on the lower floors it enjoys no natural light.

MI5 may be emerging from the shadows with its very public move to new headquarters on the banks of the Thames and the public parading of its first female head, Stella Rimington, but here old-fashioned espionage and the scent of secrecy still lingers.

There is a whiff of paranoia in the lack of open spaces and the numerous tiny offices. A large spy hole on the door to the tiny, windowless "Wendy Room" allows observers from a large adjoining room to view its occupants. In the upstairs kitchen a sign still warns "Speech on this telephone is not secure." Space is divided into red and blue zones with some areas restricted to the chosen few. Signs remind staff entering the outside world to remove their badge.

"When we first started looking around people were still working here," confides Andrew Sim, of Knight Frank and Rutley, the agents who have signed up this piece of government property which already has planning permission for a new complex of Portland stone, with shops and offices and three houses - and is rumoured to be worth pounds 70m. "With the best will in the world we couldn't get into some of the areas."

You can imagine the nods and the winks of the spooks at the Pig and Eye Bar on the sixth floor, each pretending to know more than the other. Here staff were encouraged to do their drinking on the premises rather than in the legions of watering holes near by ... to prevent loose tongues and all that.

There are also signs that it was time for new premises - the telephone dial locks hardly seem state of the art.

Now only a man from Chubb Securities protects the former heart of Britain's domestic security network. He finds it a spooky business.

"It's eerie at night," he said. "You wander down these corridors and into all those rooms and you wonder what they got up to here."

Then he smiles from behind the booth. "Still, I'm safe behind this bullet- proof glass."