It's a long shot, but it's often worth asking the parts dept at your local Jaguar dealer when something like this is showing as no longer available at the warehouse. The dealer parts system links to all other dealers and they can tell if any other dealer has one in stock, even if it's no longer available at the factory. 9 times out of 10 you'll be out of luck but on a few occasions I've managed to get hold of something that's been sat on a shelf somewhere for years. Once identified, you might have to arrange things yourself with the dealer who has it because some dealerships won't sell to others, but most are generally willing to post it to you for a charge and only too glad to sell something that's sat gathering dust. All depends if the bloke at your local dealership can be bothered to go that extra step for you and check... I've always had good service from mine in this respect.
1994 XJ6 3.2
1983 XJ6 4.2 Series III