"Where does MI5 get its cars serviced?" That question sounds like the start to one of those corny jokes you hear just before closing time in the pub on a Friday night.

But wait a minute. Perhaps an initial serious attempt at answering the question would be to suggest a depot on an industrial estate, close to the famous disused Battersea Power Station in the Nine Elms area of South West London. A substantial unmarked building at the end of Ponton Road (map below), next to the Christie's Fine Art Auctioneers warehouse, turns out to be the Government Car and Despatch Agency (GCDA).

In the late 1970s, following the demise of the Barnard Road Garages, it seems MI5 decided on a replacement location that nobody could really describe as "secret". The chosen location was on a busy South West London suburban high street with hundreds of buses and thousands of cars passing every day. It is just across the road from a popular common where people walk their dogs and next to a huge supermarket and a large popular pub. It is directly opposite both a petrol station and the area's main health clinic and is just around the corner from two nurseries where young mums drop off their little ones before going to work.

In other words, the depot could hardly be MORE public! Perhaps the reasoning was that being so public and "ordinary", the location would not attract suspicion and it would "blend into the background" of everyday life! So where is it?

Take a look at the large building on Streatham High Road (map and first photo below), just south of Streatham Common, in the space bounded by the junctions of Kempshott Road and Penistone Road (second photo below).

Streatham High Road is one of the busiest in London and is reputed to be the longest continuous high street in Western Europe. It forms part of the A23 trunk route - the London to Brighton road. The area around the northern end of Streatham High Road was previously infamous for a 1980s scandal that rocked the "Establishment" and inspired the hit film comedy Personal Services. It was the location of "Madam Sin" Cynthia Payne's brothel, where she supplied exotic entertainment for Members of Parliament, High Court judges, senior policemen and journalists - all in exchange for a fist full of Luncheon Vouchers.

Eagle-eyed viewers of the BBC's Top Gear motoring programme in July 2006 would have spotted the Streatham MI5 Garages making a surprise guest appearance (above). In one of the show's regular challenges, presenters Jeremy Clarkson, Richard Hammond and James May attempted to construct a Caterham Seven kit car in a workshop bay at Knockhill Racing Circuit near Dunfermline, Scotland. Meanwhile, the enigmatic racing driver and resident car tester "The Stig" drove a ready-built production vehicle from the Caterham factory showroom in Surrey up to Knockhill in the hope of arriving before the presenters could finish their assembly work.

In a brief scene at the start of his journey, "The Stig" was shown driving along Streatham High Road right past the MI5 Garages! But did he pop in for a little performance tweak? If he did, it was all in vain because he was stopped by speed cops just a mile from the Knockhill Race Track and so lost the challenge. Little did he know that just a few hundred yards away was another Secret Base hidden in a Scottish forest clearing! To find out what it is, be sure to visit my main Secret Bases website!

Rather than a top secret MI5 agents' operations base (details of which publishers are obviously requested not to disclose, under the UK MoD DA-Notice system), the Streatham depot is simply a facilities workshop where Security Service vehicles are serviced.

It may even be used to "lift" target vehicles, say from public car parks, fit them with tracker and eavesdropping equipment, then drop them back (suitably "altered") where they were found! It is of course imperative that the A4 engineers make sure the driver's seat and rear view & door mirrors are in exactly the same positions as before!

It will come as little surprise that Duncan Campbell - one of the leading independent experts on "Secret Bases" in the world - has written about the Streatham depot on several occasions over the last 25 years. A decade ago, he was even interviewed standing outside the main gate, in a TV programme hosted by Chris Tarrant - now more famous for teasing and taunting contestants on the UK version of "Who Wants To Be A Millionaire?"

Mr. Campbell tells me that the Streatham depot may even have been used to kit out the vehicles required by MI6 handlers to hurriedly remove their long serving KGB double agent Oleg Gordievsky from Russia over the border into Finland in 1985, hidden in the boot of a car.

The Streatham garages and many other past and present facilities used by both MI5 and MI6 have been documented by Richard Bennett, the author, broadcaster and photo journalist who maintains his fascinating AFI Research (American and Foreign Intelligence) ISDB - International Security Database.

Therefore all these locations are already widely in the public domain - it's impossible to "force the genie back into the bottle", as the saying goes!

It was ironic that for once, I hadn't needed my usual "Secret Base" research tools - those trusty Getmapping aerial photos. MI5 had used a form of stealth technology that comes very cheap. They had been brave and bold and placed a "Secret Base" where you'd least expect it to be - "in your face" and "right under your nose"!

Just like the old 1970s Barnard Road location, the same has now happened with the Streatham garages. The story has gone full circle. The site has long since been vacated. According to their official website, MI5's Watchers have moved to somewhere else in the South East of England, not so "in your face" and "under your nose".

But in 2008 I discovered that precise location and also another newly constructed major MI5 vehicle workshop and rapid response unit somewhere in "the North". I have all the architect drawings and they both appear on Bird's Eye aerial photos. But for obvious reasons of national security, I am not describing them in detail.