Solution - there is a ball-bearing used as a one way valve built into the inlet manifold where the pipe is for the return to the expansion tank - its only purpose is to aid warmup of the engine from cold. when the engine gets to temperature the pressure moves the ball-bearing and allows water to pass back to the expansion tank. this ball-bearing over time gets clogged up and sticks in place preventing the water to circulate when it gets to temperature.
first drain the coolant by removing the bottom hose from the radiator. then you need to remove the manifold you will see the offending design flaw on the left hand side. then using some rags clogg up the ports on the head and the manifold to prevent any of the swarf entering the engine. use a 3mm drill bit to drill out the area retaining the ball bearing. this will allow the ball bearing to drop out. use a 4mm drill bit and carefully drill right through the pipe to increase the internal diameter. bolt it all back together refill and bleed the coolant system with the heaters fully switched on and engine running.
Happy days no more overheating issues.
note: probably a good idea to flush out the expansion tank and radiator and the internal heater matrix (2 pipes that go through the firewall) using a hose pipe. remove both pipes and run a hose pipe through it till water comes out of the other one clean.
