LPG won't switch once warm

Hi guys,
My classic RRC is being a pain again!

It will start on petrol fine, flick to LPG and run fine until I switch it off then it will start on petrol fine but won't run on LPG till it has cooled checked the vaporiser etc and its warm to touch. Coolants at correct level.
Any ideas ?

It's a old EMMEGas system single point thing

Vapouriser needs to be at engine temperature, check the coolant hoses and if possible route water flow: engine > vapouriser > heater > engine and do away with the Y or T pieces.

Change-over on our D2 is normally when coolant is warm-ish, but we can manually start on gas if we want to by holding the selector switch in the Gas position, wait for the gas solenoid to click and then start.

Peter