I've been having fun a games with the replacement head. Two of the rocker cover bolts had sheared off (not my doing) so I tried drilling them out with my crappy 50 quid pillar drill. Needless to say, they were a bit out but I tapped them anyway.
After refitting the head, cam etc those 2 bolts wouldn't go in because the holes were a bit out.
I then took the head to an engineering bod who does engine work. He spark eroded the old bolts out but couldn't re-tap to M6. Instead, I get my head back tapped to M8. I wasn't over happy with that so I had a bit of a think.
I bought a length of 8mm aluminium tube from B&Q and tapped an M8 thread on the outside. I then cut two 20mm lengths and threadlocked them in the M8 holes. After leaving it a bit for the glue to go off I tapped the middle to M6, ready for refitting - again.
Getting there, slowly.