In reply to dunc56:
Thanks everyone for your help. I'm currently around 95% sure I've got the plate and about 5% sure it's another variation. It was actually your tip Dunc that helped me figure it out. Even though the analyser was chucking out random plates it couldn't possibly be, it suggested one letter which let the whole thing be decoded.
In terms of backstory I can't really say too much, but as I'm sure you've guessed, this car was involved in a crime and we are trying to track it down.
For safety reasons I'll probably request this thread deleted later, but I wanted to thank everyone for their help, without which we wouldn't have made some positive steps. So cheers all