If you go for a replacement main thermostat, ask for the coolant temperature sensor to get changed at the same time, it mounts into the thermostat housing and is nearly the same amount of work to install as the thermostat itself - it should be a part only cost as old one has to be moved into new thermostat.
I replaced my thermostat and after 6 months I had to replace the sensor to stop bad fuel consumption and fans running nearly constantly...
Kind of wish I could have told myself the above, fuel saving would have easily paid for the sensor!
If you are quick, inline thermostat can be swapped without loosing too much coolant.