It does seem worse now plus I'm losing a few mpg I think
The thermostat usually opens at 90 degrees causing coolant to circulate around the system and therefore cooling the engine. It only opens at 90 degrees to allow the engine to warm up to effective operating temperature quickly.
If a thermostat is stuck open (as it sounds like yours is) then coolant is constantly circulating meaning your engine is never getting to the correct temperature unless you are stationary. When the engine is cold the fuelling is adjusted to compensate which is why your mpg will have dropped.
