[OzXFR]

Do you really want a working start/stop system?
Many including yours truly can't stand it and have permanently disabled it.
Easy enough to do and the best way is to completely remove the aux battery.
This effects nothing other than the start/stop system and saves you the cost and fitting hassle of a new battery.
Also there is some anecdotal evidence that a dead/weak/failing aux battery hastens the demise of the main battery.

 

[OzXFR]

Yep, just yank it out!
Dunno exactly how it is mounted in the XF despite having ownrd two of them both with these aux batteries, I never paid any attention to that detail, but in my F-Type there was a little faffing about getting at one or two of the mounting bolts (the aux battery was mounted to a large bracket which in turn was mounted to the floor of the boot) and I needed a wobble socket to access them.
Edit - had a senior moment there, of course both of my XFs were 2010 models which did not have stop/start let alone aux batteries, the XF didn't get stop/start with an aux battery until 2012!