I think my thermostat was sticking - yesterday the temp went very high, and then suddenly dropped back down to half way. I imagine if it's sticking closed it would also be pretty sticky open as well.
Fitted a new stat, and certainly getting a lot more warmth out of her.
You can check the function of thermostat by placing in a pan of water and heating slowly whilst monitoring temperature. There will be a temperature at which it begins to open and another where it's fully open, usually close together. It's possible to buy a thermostat which opens slowly though, check with your local motor factor.
I have a nominal 90°C one which starts to open at 65°C. Ironically this allows the vintage diesel in my narrowboat to run cooler than with the original 72°C one.
