I am (very!) happy to say that I will be picking up my repaired XK tomorrow. A careful dismantler with them was finally found, which only leaves me with the big bill to pay.
However, if Jaguar don't restart stocking these pipes, it is a problem that other XK owners are going to face in the future. I hope some company can be found to take on the production of these pipe sets.
Now I am off to enjoy my wonderful car again...