My bad, I really should have written up my findings. The issue is now resolved. I ended up replacing the lock unit. When doing so I found out why my original lock wasn't working.Just love it when the person with a problem comes back to tell us what fixed the problem or if it is still the same.
I had something similar happen the day of the "Beast from the East" snowstorm a few years ago - when I opened the boot to get the snow shovel out to help another driver get moving again in a blocked single-track road around 400 yards from home that took 3 hours to clear ; afterwards the tailgate just bounced. The repair in the end was replacing the tailgate solenoid - got one for a few quid off here (possibly A2Steve but certainly one of the A2oc parts suppliers) ; then underwent the enjoyable process of learning how to remove the irremovable tailgate internal trim panel using A2oc tutorials and "tailgate internal trim removal" threads and a moderate amount of increasingly-brute force, then actually did the removal and replaced the solenoid and got the tailgate working properly again. We have another solenoid that is beginning to get a bit lazy (will latch if you drop from 2 inches but not if you whack it or drop from 6-inches or higher) so this will probably happen again fairly soon. Tailgate trim is now an arduous but not difficult job, which I suspect is about as easy as it is for anyone at the very best of times given how securely they are affixed if not meddled with.I had a similar issue the other day. Hatch would not lock and bounce. @Menno had the amazing idea that it might be the lock cannot catch the hook. He moved the hook a tiny bit upward and that did the trick. You only need a 10 or 13 wrench (not sure what size, but it is in the cars equipment) to loosen the bolt that holds the hook, move it a bit, and retighten.
Maybe this will help.
Goodluck and a safe trip home.