I came home tonight and the wife reported "I can't close the boot". Sure enough, it just bounced and pushing the 'open' button on the tailgate resulted in a brief 'drilling' sound. No amount of tapping, hitting, whacking, locking, unlocking or anything solved it.
So I took the lower tailgate trim off (what a pain that is, but eased by an excellent diagram I found on this very forum) and discovered I don't have a socket suitable for getting the boot latch off. A dash to Halfords remedied that (it's a splined-drive type, but a T40 Torx undoes it) and so I got the latch off. It is a single unit comprising the latch and opening 'solenoid' which is sealed... but not sealed enough to stop me getting in! On disassembly I found that a plastic worm gear had sheared, which rendered the whole opening mechanism useless and had the unfortunate side effect of failing the latch in the 'open' state, hence the boot wouldn't shut.
The 'solenoid' is actually a motor that turns a plastic worm gear; this worm gear slides a plastic.... slider.... along to pop the latch mechanism open. The motor stops when the slider reaches its limit and there is a spring that winds the gear back again when the motor stops applying torque. The drilling sound was this motor spinning the broken worm gear and grinding the broken parts together.
The only solution is going to be a new latch unit... after posting this I will be trawling the 'Net for a part number and possible supplier. Failing that I'll have to call a dealer tomorrow. Until then I have managed to remove the broken worm gear, meaning the latch will close but we now can't open the boot without climbing in from the rear seats and operating the emergency unlock. At least it is secure and safe.