Thanks for reminding me of this film. (It's the one where he keeps himself awake by driving a pin into his palm, yes?) It's very much paired in my mind with the TV series Callan, which started a couple of years later, and starred Edward Woodward. I wonder if Callan was in any way inspired by Deighton? Only that was even darker and shabbier more morally ambiguous, with the title character's sidekick being called Lonely for reasons of (iirc) personal hygiene. I think they made a film of it a bit later, but I'm not sure how well it stood up to the original series.