ever since that time of unbound paranoia the one unbreakable law of the British secret services has been: Thou shalt not snoop on Number Ten. Because we are not in the business of generating policy--it's not a task for which agencies like ours are suited, and in those countries where spooks set policy, it always ends in tears. We vet politicians on the way up--that's an entirely different matter--but by the time they're moving into Number Ten they should already be above suspicion; if they aren't, we haven't been doing our job properly. And