Bar's Stop Leak - $15 per bottle -- Part No. 1186 - Liquid Aluminum Radiator & Heater Core.
Bought this on the recommendation of my car club to address a potential heater core leak; slight mist on windshield. I believe Jaguar also recommends this for earlier models, i.e., the XJS.
Bought a case of six, sold one, Used one, and am keeping one as a spare, so there are three available, for $15 each.
Instructions are on bottle. Product seems to work.