Don't know that model but have had extensive experience with flat batteries
on XJ6 models. First up, there is a lot of computerised etc. equipment in
Jags, and unavoidably it takes a small drain. A good battery should be OK
provided the car is used say once a week. Overnight flattening is much too
fast and may indicate a very poor battery.

Next possibility, an aftermarket alarm. They often draw absurd amounts of
juice when in the ambient condition. Check anything non-standard and added
to the car.

Third possibility, boot (trunk?) light staying on, or some other light.

Scientific test. Place analogue (not digital) multimeter is series with
battery lead (when battery is charged) to measure quiescent current. DO NOT
ATTEMPT TO START CAR WHEN YOU DO THIS. Ambient current for the XJ6 was under
1 amp from memory. If much higher, something is wrong.

The reason you need an analogue meter is that there will be a steady small
variation owing to the clock pulses. On a digital meter that would make the
digits constantly change. On an analogue meter the pointer just sways a
little.

Hi Guys
Just to let you know after visiting 3 Jag "experts" the prob was
identified as a faulty air con sensor switch, which drained 5 amps per
hours (always) compared to the ideal 0.5 amp an hour when activated. Cost
£70 to identify and rectify so all well now. Many thanks to all who
offered advice.