Thanks for the replies. I quite appreciate that an unrecorded internal part cannot be noticed but it seems that the annual check should have flagged the fitting of an incorrect engine, if not the other non-timed time expiring parts.
It is interesting to question whether the insurance was invalidated if the aircraft should not have had a CofA, even if it had a piece of paper. I suspect the answer will be driven by whether the owner / pilot knew of the deviation.
John