I think you found it, John. Same problem on my 98 xk8 was a "warped" PLASTIC thermostat housing.
No sooner was that fixed than I had overheating and more coolant smell. That was disintegration of the PLASTIC impellers in the water pump.
After those fixes, no problems last 2-3 years.

 

My water pump was replaced before I bought the car the previous owner gave me all the receipts and it says the replacement has a metal impeller. I had the same problem with my old Merc Sable the water pump had no impeller at all when it was replaced! I replaced it with one with metal as well. The only problem you can get from the impeller dissolving is the particles can clog your heater core!!! I work on a lot of cars for friends and I hate to say it but they all use plastic and usually all that stuff doesn't have a long life! I can understand Chevy Toyota or one of the everyday brands using plastic but can't figure out why Companies like Jaguar, BMW, Mercedes, Porsche and the more expensive companies use it.

I checked with several Jag mechanics and they all recommended replacing the plastic parts with metal. Supposedly if you get your car repaired at the dealer they have to replace it with the OEM Jaguar plastic stuff.

 

My kitten is visiting her mechanic on Monday for the same symptoms. She's drinking about 4-8 ounces of coolant a week (I check it religiously every Monday), but no puddles, drips, etc. But I do get a coolant smell when I get out. I will report back what my mechanic says and the cost. Maybe it'll help someone else out.

 

I just found a leak near the sensor on the reservoir. How is that fixed?

 

the header tank can split. A life of heat/cool cycles put them under pressure.

Drain the header tank, wrap masking tape on each cooling line and mark their position for installation (this helps not make a mistake on wrong reconnection) and disconnect them and remove the tank for a better look. If it's split, it's a case of replacing. It's a fairly common failure point.

 

Prognosis from the mechanic on my coolant issue...leak from PLASTIC thermostat housing. Having that replaced with a new thermostat, a water distribution pipe and a few other gadgets/gizmos around that. $680 for all parts and labor from my non-dealer Jag specialist.