The XJR does have a different housing, just looked up on parts manual but not sure what it is made of, I do think you need to be certain it is actually overheating, the coolant temp sensors on these can go bad and cause an overheat warning, mine did this, i would let it cool and start up it would be ok till I moved off then it would go right up to the red, however it was not overheating at all, just the sensor, another test is to check for bubbles in the coolant tank whilst running the engine with the cap off, this can indicate a head gasket problem and yes it could be the thermostat but don't be tempted to try running without it, some vehicles will overheat without due to bypass (not sure if this is the case with the XJR.

 

$12 thermostat and gasket and it's back on the road. All seems good now!
Thanks everyone once again!

 

Jaguar's Premium Cooling System Fluid #WSS M97B44-D is compatible with Prestone's Dex-Cool Extended Life Antifreeze Coolant.