Here is my current dilemma all you learned folk. Hopefully there is someone that may be able to assist overcoming this little problem;
How can I gain access to the battery/open rear hatch?
Simple question. Is there a simple answer?
Cheers;
John.
I’m surprised Jaguar didn’t put a jump start terminal in the engine compartment. The easiest, safest approach I can think of is to get a battery charger that operates through the accessory/cigar lighter port and get some juice into it that way.
If the battery isn’t ruined, you can build up enough charge to pop the hatch. Such a charger might supply enough current to power the hatch release outright.
Another way is to find a large positive lead somewhere in harness, such as leading into a fuse box, and then connect a charger there (using some suitable point as the ground for the charger negative.)
Or hire a locksmith to pick the hatch lock, if there’s no way to get into the hatch to pull the emergency escape handle (found at least in U.S. market cars anyway.) From what I recall the X150 boot is sealed by a bulkhead from the rear seats.
But how are you going to do anything with the car without a physical key?
Dave
Hi Dave,
All Great suggestions, only thing is the cigarette lighter accessory plug is disabled when the ignition is off, so that would be a workable solution in this instance. Thank you.
But a bit more research I’ve discovered that there is a remote positive terminal tucked right up in the inner guard behind the near side rear wheel, with a black rubber covering. 
So obvious, its a wonder I didn’t see that in the first place!
Anyway, connected a charger on that now, so will hopefully be charged by morning. But I think a new AGM battery will need to be next.
How do I manage without a key? Well, I’ve never had the need for one as it is all keyless entry.
Thanks again for the suggestions Dave. Will keep them up my sleeve for any other similar events that may arise. 
Cheers;
John.
John, I just changed the battery in my 07 XKR and here’s what I learned.
You need to remove the cross member in front of the battery to access the hold down bracket bolts.
The battery weighs around 60lbs and is almost impossible to remove because of the reach. I made a plywood platform that I laid across the spare wheel tool carrier so I could climb into the rear and then pull out the battery.
I bought the Interstate AGM Group 49 battery which is supposed to be one of the better ones and cost around $250.
Andy
John,
On both our 2016 XJL and our 2012 XK , there is a key in the smart key fob . Slide the side off and the key is there for you to use .
KR.
Haha, yes that would be great. But as mentioned, I didn’t have the key, as my FOB contained a blank. So I now need to work out how to get one cut, given that I don’t have an original to copy. 
Wouldn’t a Jaguar dealer be able to cut a key based on the VIN of your car? I don’t know if modern Jaguars ever included a key code in the paperwork when buying new (all my modern Jags were purchased second-hand, so could be missing some of the paperwork the original owner would have received.)
Dave
Yes, I suspect so. Will enquire and be prepared for them to empty out my wallet. 
Have you tried taking the car to a competent locksmith? A friend lost his only key and a locksmith was able to cut a replacement by removing the lock and ‘seeing’ the tumblers.
2007 XKR, USA, owner. Folks, get down and scoot on your back to get a look at the underside area behind the left rear wheel well. I’m referring to the area just forward of the tail lights on the underside of your vehicle… You should see a single black wire about 3/8th inches think that terminates a good foot behind the rear wall of the well. This wire typically has a rubber boot that covers the end of the wire. Pull this rubber boot back and you’ll see a bolt. This bolt is insulated from touching the metal and is a positive lead from the battery. You can jump/power the car using this bolt and any grounding point close by. Also, this bolt runs through the trunk floor. Inside the trunk, remove the left plastic vent plate and there you’ll see the other end of this bolt where it attaches to the frame. Keep in mind, this bolt is insulated. I use this bolt to hook up the disconnect plug for a battery tender. If you have a shop manual on CD it is termed connector BS001, jump start post.