Think i know the problem , my car first had coolant problems because some complete idiot put the wrong fluid into the car!
( before i owned it ) So my mechanic flushed the system and also the thermostat was changed, he did say that he might not have gotten all the contaminant but at the moment it seemed ok, last night when i opened the rad fill cap and shined in a torch there was an oily like film on the top ( not coloured ) you can't see it unless you shine a torch on it, so i assume some contaminant is still in the system and is now affecting the new thermostat! i must admit its no were near as bad as it was but it may get worse.
So what now? leave it and see or once again have the whole system drained and another thermostat fitted!