You are right dandy, hot from engine into rad at the top. Cooler from bottom of radiator back to engine. BUT then to thermostat, and that IMHO the thermostat position is the problem.

Have no knowledge of the MGF, but looked through that post,the thermostat is on the inlet side of the engine.He modified it to go on the outlet side. Its a shame that he didnt log pressure in the cylinder head too as this may have prooved intresting.

Could the position of the thermostat be part of the problem?
Rover had head problems with the K series engine that was caused by the stat position