Ok,

Fully cleaned up the head, making sure none of the oilways etc were blocked with crap. Cleaned the springs, retainers, rockers, lifters and lapped all the valves in to a nice gastight finish.

Dummy built the engine back up with some old bolts n old headgasket.

Put a disc of good old playdough (yes i've got kids lol) on piston no1 and turned it over a few times by hand.

Took the head back off and no marks were on the playdough so I assume the valve/piston clearance is fine.

Rebuilt the engine fully and topped up the water. It started up straight away as before.

Let it idle for 30secs or so just to check things seemed ok then slowly raised the revs to about 2k. Seemed fine so I upped it to 3k. on the way up it sounded like the worst case of detonation u can imagine, ie about as noisy as a diesel. When it got to 3k though and I held it there it's fine.

So it'll hold steady rpm but doesn't want to run correct under load. Hmm EFI doesn't have a dizzy I can turn lol.

I'm going to have a more thorough look at things again tomorrow. Just wish it was something simple. hmm if nothings obvious then i'll tax the old girl n take her to a garage n check mixture etc.