Good Investigations by Ben Westerham

David Good, a morally confused and womanising private investigator in 1980s London, is hired by a ridiculously beautiful blonde to help her fend off the attentions of a serial blackmailer. He's barely got to grips with the woman's keen sense of self-interest when he stumbles on to something far more sinister.

Despite the obvious risk to his own carelessly maintained health, he can't stop himself going out on a limb to help those who can't help themselves, though his trade-mark sense of humour might not be enough to see him safely out the other side of this particular engagement.