Group,
Had an interesting problem the other day, in that I suddenly lost all heat from the heating system. I have solved it but thought the info might be useful for others in the group.
After some investigation I found that the fuse for the front fog lights/security sounder had blown, on replacement it blew again immediately.
It turned out to be the security sounder had gone faulty, and disconnecting it, once I found it (tucked down the nearside edge of the radiator), solved the fuse problem, which also to my surprise cured the heating problem as well.
So if your heatingt goes cold, check out the front fog light fuse first and if blown the sounder is the most likely culprit.
Jaguar took a swift £90 off me to replace and all sorted.