As you said you have only installed new wp and thermostat due to leaking wp, so you never had a over heating issue prior to this. The only possible cause that's coming to mind is an air lock In the system, my suggestion to you would
Be to run the engine with heater on and rev the engine at about 3000 rpm or so for a few minutes giving the engine a break every now and then in-between, with the expansion tank cap off. This should Get rid of any air locks you have (hopefully), if This didn't help check for correct operation of the circulation pump, just feel it with your hands and make sure the motor is running. If this was also working and you were getting the heating issue then feel around the radiator all around an make sure you don't have any hot spots, if the temp is the same all around then there is nothing wrong with. All these tests should bring you to a conclusion, good luck