Here the spirit of Smiley has given way to that of Arnold Schwarzenegger. The edifice is all pectorals and gigantic shoulders, enveloping a button-sized, coiffured head, in the form of its central rotunda. A jagged assembly of steps and semicircles, its movements are robotic, its speech deliberate and its skin-coloured, fleshy protruberances thinly conceal a metallic inner structure.
It suggests, too, a spectacular public event, with its fearful symmetry and regimented conical trees, clamped in place with heavy steel brackets and clipped to within an inch of their lives. Yet public displays are the last thing one expects of MI6.
How has this dramatic mismatch of building and user come about? One might detect the traditional civil servant's delight in sending other civil servants to benighted spots such as Croydon or Canary Wharf. The most likely reason is the sort of hard-headed financial decision that involves occupying offices in 1994 at 1988 prices, which was when the Government took the building on.
These matters, however, are between the Government and the taxpayer. What matters to architecture is this: whether the MI6 building is a fit conclusion to more than 10 years of agonising about a site which, according to Michael Heseltine in 1981, was 'as outstanding an opportunity as is offered anywhere in the Western world, straddling as it does a major approach to the capital'.
The traffic-swept area is the most embarrassingly dismal corner of central London, but, being large and on the river, the site offered the chance for something remarkable, a new Adelphi, say, or architecture of the quality of Wren's and Hawksmoor's further down the river at Greenwich.
Its story starts in earnest in 1979, although it already had a 20- year prehistory of unrealised proposals. In the year of Mrs Thatcher's first victory, Keith Wickenden MP, visionary Thatcherite and founder of European Ferries, set about realising his beliefs.
He wanted to prove that enterprise and culture could prosper together, so he commissioned a scheme for the site (which his company owned) that would include not only 300,000sq ft of speculative offices and 100 flats, but also a stupendous gallery to house the Tate's collection of modern sculpture. The architects for the scheme were Abbott Howard, whom the Observer identified as the ones to watch out for in the Eighties.
Unfortunately, the gallery would have occupied most of the site, which meant the offices being housed in a 500ft tower in green glass, soon nicknamed the 'Green Giant'. The design caused outrage. At a public inquiry it was called a 'monstrosity and a gross overdevelopment', the inspector found against it, and Mr Heseltine, in his first stint at the Department of the Environment, refused it planning permission.
European Ferries sold the site, Abbott Howard retreated to a more peaceful life in Hertfordshire and Mr Heseltine called for 'the highest quality of design', and an architectural competition.
One followed, for the Green Giant site and vacant land on the opposite side of the Vauxhall Bridge approach road. Some presentable designs were produced, even though the developers' brief still called for an awful lot of offices. Shortlists were announced, public opinion canvassed and ignored, and a winner chosen. The new owners said, 'This is one scheme that will happen' - and went into receivership.
Eventually another developer, Regalian Properties, bought the Green Giant part of the site, and employed Terry Farrell as its architect. His scheme had arranged 'itself regularly like beads on a necklace around a central public spine'. It created public spaces, serving a mixture of flats and offices that were sheltered from the area's busy roads and opened on to the river. It promised to create the beginnings of a neighbourhood, something the area conspicuously lacks, and to make the Thames more accessible.
But in 1988, after Regalian had appointed Farrell, it decided that a very large office block, uncontaminated by other uses, would be the most desirable use for the site. Farrell designed one, and the previously hostile London Borough of Lambeth, sweetened by a 'planning gain' - a large sum to spend elsewhere in the borough - granted planning permission. The scheme was for 50 per cent more office space than the Green Giant, but there was no murmur of 'gross overdevelopment'.
The Government then bought the site and projected offices for the use of MI6, and had the design made more secure. Windows were omitted from the bottom two floors and a dry moat and a wet moat - the latter disguised as a fountain - were constructed to keep the public away. All that remained of Farrell's necklace of public spaces was a riverside area divorced from any other form of life and thus of limited use to the public. And 13 years on, the Tate is still looking for a new gallery.
Although the MI6 building intrudes on the skyline less than the Green Giant would have, it also contributes less to its immediate surroundings; and, while office workers in the latter would never have been far from an inspiring view, those in the bowels of MI6 can be 100ft or more from a window that does not even look on to a light-well. Farrell, champion of accessible urban spaces, of mixed use, of the humane, of knitting together ravaged fragments of cities, found himself building an aggressive, single-purpose fortress.
It was the market, he says, not his decisions, that led to this conclusion, and one should attend instead to the architectural composition of his response to a given brief. This argument, in favouring abstract formal qualities over the creation of public spaces, is an odd one to come from an avowed populist, but it is true that Farrell is an accomplice, not a prime mover.
There are, in fact, no individual culprits. One cannot exactly blame successive developers for pressing to build a large office block until they got their way. One can, however, criticise a system in which ministers pronounce about outstanding opportunities, without taking sufficient action to lend force to their pronouncements. If a large office block was, after all, acceptable, it should have been permitted at the beginning. If not, why is there an even bigger one there now?
There is also a confusion of form and contents, of aesthetics and use. The main issue on this site concerned use: whether flats, restaurants, open spaces and a gallery were desirable, how much office space was reasonable, and so on. But ever since the Green Giant was rejected, these issues have barely been discussed. The Heseltine-promoted competition, like Farrell's defence, was about appearance, about dressing up overdevelopment.
Yet if a building is too big and monothematic, the design can do little to make amends. The best architect in the world cannot dress up a mastodon as anything but a mastodon. Here Farrell's complicity becomes active, not passive, as he promotes an architecture - Post-Modernism - suggesting that you can have it both ways: that an architecture of facades need not be compromised by whatever it contains.
It also undermines the worthier aims of Post-Modernism and of Farrell's earlier work - diversity, richness, accessibility - by stitching a cloak for the monolithic. The MI6 building is as overbearing and impermeable as any monstrosity you could blame on the Modern movement. It is even made of that dread Modernist material, concrete.