I now have the engine down to the short motor and have been cleaning the carbon deposits off the top of the pistons. It hasn't been too bad, thankfully.
However this, on No. 2, is somewhat disturbing!
I suspect that this is a sign of some detonation. The bore looks (?) OK but this will need to be removed and replaced, so it looks like I am up for an octet of pistons!
Cheers
Geof