I realised a few pages into this that I had already heard an audio version - one of the first Bernice Summerfield plays from Big Finish, which had had some surgery to remove the Doctor (who isn't in it much anyway) and Ace (who I think is mainly replaced in the play by Benny's husband Jason), and co-starred Colin Baker as the mysterious Russian character. I remember enjoying the play; I also enjoyed the book more than I expected.
My expectations were low because the author is Nigel Robinson, whose prose style I find in general pretty clunky. But in fact he is way ahead of his usual output here. There were two chapters which did make me groan but also one (Benny's weird dream) which really made me sit up. I also thought he caught both Benny and Ace very well, as well as having a comprehensible plot. So, for once, a Robinson book which I rate as slightly above average.