Murder In Hindsight: Revised Edition

Detective Sergeant Kathleen Doyle was almost certain they were dealing with a vigilante, in these seemingly-unrelated murders.
Each of the victims had been killed in the quiet confines of a London park, and each time, the victim was someone who'd been suspected of an earlier murder, but who'd managed to get off, for one reason or another.
So; who was making certain that justice was meted out, even if a bit belatedly?
Meanwhile, Acton seemed to be drinking a great deal, and dropping hints that...