Thermal grease is a very good idea for the fuses.
A much better potentiometer is a brushless type. They last forever and draw less current, when turning. They are about the same cost as a Jaguar item.

 

to John_Cook12

to remove the black plastic knob from the shaft of the Dimmer-Rheostat, you need a very thin "L" shaped pick tool.

There is a hole on the side of the knob's "stem", and there is a ball on the end of the dimmer's shaft which has a spring.

you insert the pick in the knob's hole and push the ball down while you pull the knob off the shaft.

in the Series 3 XJ-6, you might not need to remove the entire panel to remove the Dimmer; If you can pull the trip computer using either a guitar string or fishing wire or even floss string, you can get to the dimmer through the trip computer's opening.

if that doesn't work then you need to lower the panel to access the rear. The reason being that the dimmer switch may or may not be bolted in place at the front.

David Boger wrote a great How To on removal of the panel:

http://www.everydayxj.com/Fuel_Switch_Replacement.php