Only a short update as I couldn’t feel my toes anymore so had to come in…
Before I went away, there was just the one piston crown left to clean decoke, so I got that done and out of the way. I then slowly worked my way around all the holes in the block and the head with the relevant taps (and some oil) and cleaned all the sludge, goo and swarf out ready for re-assembly.
Didn’t do the hole for the temp sender, though, as that appears to be a slightly conical thread 
If nothing else, I must be in line for the “slowest head gasket replacement … ever” award 