Robert Beatty's character, Fenton, is a surgeon and is therefore quite correctly addressed as Mister Fenton, so this is not a goof. In the UK all surgeons are addressed as 'Mister' 'Mrs' or 'Miss' and not 'Doctor'.
It's clearly established that Jackie Leroy lives on the top floor of her shared house, in Flat 8. Fenton, however, doesn't press an entry buzzer at either end of the eight-buttoned door panel (so actually buzzes Flat 4 or 5) yet stills gains entry to Jackie's flat.
When Fenton is questioned by the police, he says he never saw a cigarette lighter by the body, but earlier, when Fenton discovered the body and examined it, the cigarette lighter can be seen lying by the body just above the knee.
Mark Fenton is a fully fledged doctor, but is repeated addressed as "mister" by everyone including the police, and the name on his office door also identifies him as "MR." Mark Fenton.