I have never heard of a horn failing on a car before, but of course Jaguars are capable of anything. Used replacements (carrying the same failure risk) on eBay are quite expensive.
Step forward this "Wolo Maxi Sound 2" horn set (obtained from Advanced Auto) $28. Twenty minute direct swap (using ramps to get under the car, $60 at Harbor Freight) and job done. I love the finished look. They just play a tiny game of peekaboo through the oval grill if you look at the right angle. The red is awesome. Poor man's red brake calipers!. Apparently horn fittings have been standard since 1955, so you are not confined to Jaguar originals. These sound great.