I think a third party was mentioned because JM didn't want to implicate Jeremy. It usually happens that way when one looks to someone else to blame.
If Jeremy had had relatives on his side JM would have sided with them instead of the others. It's how it goes.
If Julies witness statements were false then this would make sense.
It would be dangerous to directly blame Jeremy in case he could prove he was elsewhere at the time.
This is what actually happened apparently with Malcolm.
At least with Malcolm they could put it down as the one detail that was a lie by Jeremy.
If Julie was lying then I am sure she was put up to it by the relatives.
It was her statement and the silencer that put him under suspicion.
Without those 2 things he never would have even been a suspect let alone convicted.
If DC Jones was convinced that it was Jeremy then he would be happy to help coach Julie in her false statement - noble cause corruption was rampant in Essex in the 1980's.
If anyone doesn't believe me then research why they had to bring in the PACE Act of 1984.
It was because noble cause corruption was so absolutely rampant in the UK.
The PACE Act 1984 made it a lot harder for them to get away with it.
But the reason the Act was necessary was because it was so rampant at that time.
It was almost standard Police procedure at that time.
Luckily because of the PACE Act 1984 noble cause corruption is now a lot rarer.