A little update, I seem to be at the end of the road with this one now...ubless anyone have any ideas to try.
Heres what i have:
Drive it like you stole it toasty hot engine, temp gausge 90, heater blows hot. Get back home, hand on the coolant pipe off the thermostat houseing and its cold.
When engine is cold open head tnak cap and pop, coolant spurts out. Evidence of it spurting out during driving on inner wing.
What Ive done:
Emptied the coolant, disconneting pipes off thermostat header tank and lower radiator, oil cooler refilled it got it hot, cooled it, drained it again changed thermostat for genuine audi one. Tested the thermostats old and new on the bog, both open fine.
When Ive pulled the thermostat out the engine watr fell out and flowed out.
I jetted water in every hole I could find until the outlets ran clear clear. Refilled with with correct coolant (Mannol g12+ pink)
Drove 35 mile to work some bits 80mph on motorway, got to work and pipe out of thermostat is cold to the radiator. Pipes from expansion tank to head red hot and firm, radiator top hose hot and firm.
The only thing different after flushing was the temp sender is slow sending the temp, it struggle to get to 90 now even more than before!
My feeling are theres some block in the head stopping water gettting to the thermostat when the pump is running and engine running?
Anyone got any ideas what else I can do please before my pipes pop