Thanks for your reply, it has just had a full service (air, fuel, engine oil + filter, front and rear pads) less than 250 miles ago. Well it has been stood at bmw for 2 months or more. It is also on shell v power diesel. But it's just used 1/4 of a tank so not been on it long!
I'm hoping bmw will change me the thermostat under warranty, if not i will do it myself. I will look into the 320d posts.