I would love to see which other murders from 1986 the ex cop is alluding to..I think the "3 murders in 1986" theory is far fetched, and the numberplate was random.
My personal suspicion is that JC only risked keeping the Mini because he was planning to use it to lure in a potential future victim.
The last digit in the registration is an S. That would make Shirley's car seem newer than it actually was to a potential buyer. In the UK, registrations changed each year A,B,C,D and so on (IIRC excluding I,O,Q and possibly a few others).
JMO but I reckon JC was planning for 30 young women to come round and look at the Mini he had for sale. He got 30 women to call round when he advertised for a female cleaner in Clifton.