Charge your battery and then check again. These cars do not function well with a less than fully charged battery. Always the first thing to check especially when they disappear upon a restart.
If you don’t already have it, here is the electrical manual for your car.
http://www.jagrepair.com/images/Electrical/XFElectrical/2009 MY XF Electrical Guide.pdf
There is a known issue with water from the windscreen washer pump wicking into the electrics, specifically causing corrosion in the Central Junction Box/fuse panel in the drivers footwell, which can result in all kinds of electrical issues. There is an updated washer pump and wiring as a fix but it is not a cheap job so worth checking for signs of water ingress and corrosion in the said panel. There is plenty of info about this if you do a Google search.
Worth a look.
I doubt that very much.
No. Replace the battery with a new, fully charged one and then get yourself a battery maintainer and keep it hooked up when not in use.
I did think initially, from your description, that it sounded like the alternator as it happened to me a few years ago. Good news is, it will be sorted at the dealership and you won’t have to worry about resetting anything as they should do it all for you.
You don’t need one Jon. Replace battery, reset park brake, windows and reset BMS and away you go. Or, hook up a temporary supply under the bonnet, change the battery and you shouldn’t lose anything.
An improvement sounds good though I would say you still have some air in the system. If you can live with it then fair enough, but do keep an eye on the temperature gauge if you go on any longer runs.
Park the car so it is pointing slightly uphill. Take the cap off the reservoir and then allow the car to run up to temp at idle. When the stat opens the coolant will flow around the system and the car will burp itself at the reservoir.