After that, however, starting slightly dubiously with Polyakov (Possibly a dangle) and highlighted by Gordievsky they were "winning". I don't have a citation on hand, but I believe Gordievsky stated that a set of diplomatic expulsions in the mid 1970s absolutely crippled KGB operations in the UK and the station never really recovered.
I personally just about (70/30) believe that Hollis was ELLI, it's just too good to be true with the lack of offical resolution and correlation with SONYA. If he was really a double agent, then the Russian's would have nearly had the heads of both MI5 and SIS in their control (Philby was in line for the throne).
I am currently writing a hands on (Zachtronics-style even, e.g. build your own https://en.wikipedia.org/wiki/The_Thing_(listening_device)) dialogue-y game set roughly in this time period set inside MI5, I hope to be able to capture how crap they were.