Maybe the Stationery Office. Although I don't think it's moved.
(Incidentally, Slider, the best of luck. I wanted to get a copy a couple of years ago and couldn't find how to buy one anywhere. I evntually got one from a mate of mine who was posted to the MOD in London at the time and who had some copies in his office. If you know anyone based in Whitehall they may well be able to help you).