The chief problem I've found with TSS is that the action seems to take place in two, very different, Moscows.
We're introduced to Moscow One in the first part of the novel. This is the Moscow where militia are on the prowl to rough up any dissident they find, where Bracken can't get loose long enough to brief Quiller properly, where you have to watch every word you say around the resident upravdom and dezhurnaya, where you can be blown off the street with a phone call, where you have deal with untrained cutouts because you're so scared of protecting both ends of a signal, and where everything appears to be what it is not and nothing appears to be what it is.
Moscow Two magically materializes after Croder's arrival. In Moscow Two, freedom of movement is suddenly increased; you can tag, capture, and question people without fear; you get the leisure of a minute examination of one of the vehicles that members of the elite governing body of the USSR ride in; and when the action builds to a climax, you have seven people in cars (all the way up to "G-George," for Heaven's sake!); crawling about the Kremlin like an army of ants, chattering to each other on walkie-talkies!
Either Hall wants us to think Croder's got the luck of the devil, or this is a lapse on his part. I feel that the oppressively threatening atmosphere painted in Moscow One is one of Hall's best descriptive efforts, whereas the climate in Moscow Two looks suspiciously like an effort to wrap things up quickly because he's run out of ideas.