Thanks for the advice and theories guys, all appreciated.

The belt looked fairly normal and appeared to be tensioned ok with no evidence of stripped teeth from what i can see. i'll take it off tomorrow and give it a good look.

I'm not confident in the reason for this apparent belt slip as nothing so far looks obviously wrong, and dont like the feel of the engine so i'm going to discuss with customer and have the head off to be safe. Keep the ideas coming chaps!