Well, that was easy! I didn't even have to remove the plastic panel.
From inside the boot you can just see enough of the latch to poke around with a screwdriver while pressing the release button on the remote. This resulted in a satisfying click and the tailgate released.
Few squirts of Teflon lube and the jobs-a-goodun!
Same has happened to me on a number of times. I used to bump the car down a curb which then released the catch but it failed to do that on the last ocassion so like the rest of you I dived in the back and smacked the lock with a blunt object and 'hey presto' the boot opened. I used motorcyle chain wax and it has been great ever since. It must just get dry and dusty in the lock and jam. I did call Ford and they wanted £100+ to fix it!!