It all hinges uopn Jeremy's word that his father telephoned him. If we believe this to be true then this in itself rules out a "hitman". To my mind I have considered this at depth and thought it extremely unlikely.
Not quite, Grahame.
What 'if', a hitman where involved (not for a minute do I believe Jeremy hired him), but what if....and the hitman wanted JB dead too.
He needed Jeremy to come over, so got his father to phone him. Now, 'what if'....the father's first call was to the Police. Eg: The hitman has told Neville to phone Jeremy, Neville picks up the phone and start to dial. During this Neville has a gun to his head/neck, which is where we get the marks from on Nevilles' neck. The hitman thinks Neville is phoning Jeremy, however, the phone numbers are so similar, that Neville is in fact, phoning the POLICE...NOT Jeremy. This also answers the question as to why Neville never called 999.
After a beating...Neville then phones Jeremy.