I have looked under the hood, but cannot locate a terminal to connect a power source to. I did see a Youtube video where the guy had a similar problem to mine, but he had a 5.0 Supercharged Jag, and there was a terminal end in the fuse box under the hood. Nothing in my 3.0 XF

 

Two other options. Jack up the car and remove front right wheel. Behind the wheel arch there is apparently a positive red terminal. I've not done it so not certain if it's on pre facelift cars though

Easier - just use passenger fuse box under the dash. Apply 12V positive to the red side and negative to a good grounding point on the car. Use that to pop the trunk

Goes without saying (but just to be sure) you of course can't jump start the car on a connection like that. Just enough low current to activate the electronics and pop the trunk

 

The easier option is the one for me!! Please excuse the very basic level questions below, but I do not want to screw up anything inadvertently.
I located the fuse box on the driver's side (right hand drive in NZ) in the wheel well cover. There is a thick red wire leading to a metal terminal with a nut holding it in place. Is this where I should connect the positive terminal? Should I then connect the negative to the metal locking bracket in the door jamb?
I have a 12V 1.5A trickle charger. Would that be suitable as the 12V source? My other option is the Super Mini Booster portable jump starter, but that holds more than 12V.

 

Correct. The big fat red wire is the positive feed to the fuse box. Connect the red positive of the external power source to that red one first (before connecting the black so if you slip and touch the metal of the car you don't cause a short to your power source). Then connect the black negative of your external power to a suitable metal grounding point. Not something painted or varnished - clean metal like the latch you point to. You may see a small spark as you connect it
Most likely the car alarm will start to sound at that point so gave the key fob nearby and hit unlock to disarm it

Then try the trunk button on the dash. If it doesn't work give it 5-10 mins and try again

I doubt your trickle charger will be strong enough as the dead battery will be pulling for charge too. Would prob use the booster unit but of course don't try and start the car from there. Just unlock the trunk
Not sure what your comment about the booster not being 12v. It should be if it's meant for a car. If not, then of course don't use it!

 

I connected a trickle charger with the positive to the red wire in the fusebox and the negative to the nut above the shock absorber, as the wire was too short to connect to the door latch. After a minute or so, the fob was able to unlock the trunk! I took the battery to a local recharging garage who put it on a 24 charge and now all is well.
Many thanks to all who provided advice and guidance!!

 

Good result. Keep an eye on the battery as once fully discharged it may not hold charge as well.