I spotted that too late about the replacements you had aready made in the engine bay.:bang:

It might be a bent rocker stud or one of the lower, plain nuts which secure the rocker assembly being loose. I had that latter problem a while ago. It only took one side to be loose and the engine died. To get home I simply tightened it and the engine started.

I'll check that too. My thinking is it could very well be the cover fouling the rockers. I now remember that before I found the advice about not over-tightening the the cover I may have over-tightened the cover the first time I replaced it with the new gasket. I remember thinking this thing is never going to be completely tight and now i realise that's because the soft metal was probably just pushing down under the load. Well I hope that's the issue as I reckon I can fix that fairly easily

If the cover is bowed down you could tap it back into shape using a piece of round bar or a socket on the end of an extension drive bar as a drift; but you would need to take care. It might need a bit of force but too much and you make things worse.:bang:

Well today - in the short 20 mins I had spare - I took off the rocker cover and inspected the inside of it for witness marks and sure enough there were the tell tale signs of the rockers touching the inside of the cover. So I quickly (but quite carefully) beat the cover out from the inside and reinstalled without over-tightening it. She started straight off. So now I can move on with some finer tuning.

I only took a week and a half of skinned knuckles and head scratching but we got there in the end, thanks to all your help here. I've learned a lot.