What you do to prove that police interfered with the batch of crime scene ammunition, is first of all obtain the Xray taken prior to the autopsy on Sheila's corpse was undertaken on 7th August 1985. You then obtain all the documentary evidence available in the archived data base which suggests that bullet PV'20 was a whole one. You arrest the bullet expert and interview him under caution about his reception at the lab' of the replaced PV/20, and you then start to interview or obtain witness statements from the pathologist and the ballistic expert to try to find out at what stage someone who was part of the investigation team swapped over bullets...