The Thursday Syndrome — Tony Nash

A corpse wearing another woman's face and using her name has DI John Hunter and his wife Jane chasing dozens of confusing leads. A phone call out of the blue offers the name of the killer of not only that woman but also of John's foster mother more than twenty years before. Unfortunately, the caller is shot before he can give any information. His death is useful, because he is identified as a spook, and his death points John towards MI5, where he is sure the answer lies.
The leads are confusing, but John eventually realises who the killer is, something he should have worked out years before.
The man is deadly dangerous and clever enough to get off scot free if arrested.
John has a deadly decision to make.