Turns out I do!
Went to change the rear pads today, only to find both pistons seized in the rear caliper, and a lovely stress fracture running through the entire thing so I've had to bin it grrrr.
Issue now is I can't find another one to replace it with, and I'm not buying from the states as I'm UK based and would pay through the odds thanks to our lovely Government.
Anyone have any idea what caliper would fit a 98 XJ600S?
Cheers