Thanks for the advice, cafcpete - and that link to the jaguarforums.com website.
So, to bring this story up to date, I tried pressing down on the boot lid whilst working the key in the lock - but all I succeeded in doing was bending the key... Having run out of options, I was finally faced with drilling the boot lid behind the number plate. The procedure I followed is explained here:
https://jaguarvp.blogspot.com/2013/04/x ... ccess.html In fact that part was remarkably easy. All I would suggest with hindsight is drilling the hole an inch or so higher - I had to reach up at quite a steep angle for my screwdriver blade to find the latch.
Removed the solenoid / actuator and tested it: nothing. So, I ordered a s/h solenoid off ebay; waited a week or so for it to arrive; fitted it, and - with bated breath - pressed the boot release button. Nothing! Tried the key fob button, and the button on the dash. Still nothing! Made a visual inspection of the wiring loom where it fits around the RH boot hinge, but that all looked fine as far as I could tell.
At this point I booked the car in with my local auto electrician. Picked it up again on Saturday, with the boot release working once more! Apparently there had been two wiring faults: one at the boot hinge and another at the alarm module. I'm now a hundred quid + VAT worse off, but at least I can access the boot again. And - for the first time ever - I now have working courtesy lights in the luggage compartment!