Gasket was blowing between cylinders 2 and 3 pressurising the cooling system. gasket hadnt blown, head was slightly warped there, 0.15mm, 50% above maximum allowance.
Bought a new head off ebay, seller had 4 for sale, but only one left when I bought one. Nice to know my problem isnt unique. Still reassembling the engine, this job requires lots of time and patience. Definate pia.