The original Jaguar part number is C27186 and the part is still readily available from many of the usual suspects and costs anywhere between $50 to $100 (even for a genuine one).
Only issue (and the reason for this post), is that you need to transfer all the fittings, including the push rod over to the new one. And its not easy.
The complication arises when you need to swap over the push rod. Removing the old push rod is a bit difficult. There is a metal collar that holds the boot and obscures access to the circlip that first needs to be chiseled off. Once the push rod is removed, the trapped washer on it won't fit into the new master cylinder. It needs to be lightly sanded down around the edges. Make sure its loose enough to slide past the circlip recess in the new master cylinder.
Now the delicate part of transferring the new rubber boot. It slides off the new push rod easily as there are no restrictions but the old push rod has the integrated fork for the clutch pedal. I managed to push the old push rod through the new rubber boot, fork side first. It is possible but it can easily tear if you're not careful.
The new master cylinder also has a wide black rubber band over the boot to hold it down tightly. This band (as shown transferred over in the photo below), cannot be used and should not be trasnferred over as it will prevent you from installing the master cylinder into the car. Which I later discovered when trying to bolt it into the car.
Make sure to use new copper crush washers and clean up the sealing surfaces of any old parts being transferred over.