Thanks for the tips.
I pressure tested the cooling system to 20 psi . It gradually leaked down to 10 psi over 15 minutes. Unfortunately I can't see where the coolant has gone.
The pistons are clear of cololant.
There definitely is a major coolant leak somewhere as the camshaft and rockers are covered in a brown/yellow creamy substance (however the oil on the dipstick looks o.k.). Even when I clean this off , it comes back with 2 minute of running.
I assume this indicates a problem with the head as no coolant has gotten into the sump yet.
I take it I need to remove the Head again and get it crack tested ?
p.s. the car is backfiring and won't idle - I assume this must be related to the coolant problem ?