Its a thermostat after all, its filled with an oil which leaks eventually apparently so replacement bulb is minimum to fix the whole thing when a quick clean doesn't work.
1996, 3.2 SWB X300 - Finally a rust free daily driver
1997, 4.0 LWB X300 - Unlucky car.
1988, 3.6S XJS - Never ending restoration project.
1994, 3.2S X300 - Accident damaged, October 2015. Gone!
1994, 3.2S XJ40 - Long gone.