From "Tinker Tailor Soldier Spy: Flushing Out the Mole (#1.7)"
You know, George, one thing perplexes me more than anything else about the mole conspiracy: well, Karla conceived Operation Witchcraft primarily as a means of placing poor Percy Alleline on Control's throne. But why didn't Karla want Haydon to simply take over the Circus himself. Well, surely it would have been much easier to arrange, with all of Bill's acknowledged accomplishments.
[cut to Haydon and Smiley walking the grounds of the detention camp
No, no. It was a perfect setup: Percy made the running, I slipstreamed behind him, Roy and Toby did the legwork. Being in charge would have bogged me down. All the admin, the dinners in Whitehall, hobnobbing with the Set...