Bit of an update, been out most of the day so not much progress.
Richard L - only one valve is involved, the other three are OK
Trev - I don't have another (alloy) head to compare and with a bit more thought and inspection the valve is just touching the edge of the pocket, if the seat was the problem then the valve I think would still go in the pocket, this valve is missing the pocket by a whisker.
Also the original pistons (9.5) - one of them does have a very slight mark on the edge of the pocket, hardly discernible but very possible from the same problem, these pistons only did about fifty miles and had started to pick-up (re bored to tight).
One common comment from most of you concerned the valve looked large, now when I started this project I had a big valve head to go on it, I wonder if I ordered big valves for it, only thous in it as I understand, but it's only a thou or two that the things catching by.
The valve when rotated in the head does not show any variance, however I have two new valves ordered from C&D, should arrive tomorrow.
AFAIK an A10 piston is an A10 piston and the higher compression ones would normally only be used on Rockets and so forth so you would presume the pockets would so sized, you never know though.
One thing I know is I will never run a project like this again, buying stuff as and when it comes up at a good price to use in the future can backfire, no I reckon buy it as you need it then you will know better what you need - you live and learn.