I have found a control module with a 6V lead acid battery attached to it.
It's located behind the rear seat back cushion.

What is it?
I can't find a mention of it in any of the service manuals I have.

The 6V battery is dead as well (1.78V)

The only electrical problem I have on the car is the windows both losing memory - have to re-calibrate regularly to get the window to go back up after closing the doors - could be related?

Tony.

 

Tony.
My guess is that it is a "Tracker" or similar. That's where I found mine when I replaced the rear speakers.
Do you have any documentation to show whether any sort of tracking system has ever been fitted ?
Could be related - that dead battery will constantly be trying to recharge and draining (admittedly not by much) the main battery which may be just enough to drop the level below that critical point where various bits of electrikery start to misbehave - particularly the windows.

 

One of the modules for the OEM Motorola phone kit resides behind the rear seats - check if you have the phone cradle in the armrest to see if it could be one of these, although if it is I would expect it to have a Jaguar part number on it.

 

The battery on not original equipment though, something must have been added by a previous owner.