Timing belt was intact however he had been having a few mystical intermittent running issues which were mystifying me.
Ive yet to do a post mortem on the head but I reckon it's dropped a valve, possibly by breaking a valve spring. Ooops.
Sourced a replacement engine for £100 on ebay and put it in on sunday. I just need to get the correct timing belt for it tomorrow and it'll be sweet.
It does also have a silver lining as the old engine has donated its wiring harness to my similar Vectra which has just been diagnosed and found to need the engine rewired. Therefore I get a free fix. :-)