Other film influences on this?
When I first watched this(about 4 years ago on Film 4 in the UK), I noticed some similarities to two other films I had seen. The first was '10 Rillington Place'(released a year before Frenzy and featuring a serial killer and the same grim London - it even gets mentioned in Frenzy). The second was 'Peeping Tom', not just because it features Anna Massey, but also because the lead actor (Karlheinz Böhm)in that looked similar to Barry Foster in Frenzy. It was that whole blonde blue eyed thing going on. Plus, Peeping Tom was about a serial killer as well. The colour scheme seems to be the same in both Frenzy and Peeping Tom.
I think that Hitchcock and Michael Powell(the director of Peeping Tom) were friends. I think Hitchcock even suggested Kim Hunter for Powell's 'A Matter of Life and Death'. I know that he thought that Powell was badly treated by the critics over Peeping Tom as well.