If a 2000 XJR has had a new thermostat, but is still on the original water pump and tensioners, is it likely that the car overheated enough to cause the impeller in the water pump to disintegrate and the tensioners to degrade?
It is normal practice to replace the water pump (when the stat is done) as the early ones can break up at any time.
Tensioners - change them, peace of mind after 8 years. They can let go at any time. Sometimes you get a warning, sometimes not.
ex 2001 S Type 3.0 SE Auto
ex 1997 X300 3.2 Sport Auto