Some good news... I've got the back seat down and replaced the missing headrest!
I did what's described above – but couldn't release the catch with a screwdriver (not enough space between seat-back and wheel-arch – or more likely not enough dexterity on my fat-fingered part) so i made a device to
help. I used a length of aluminium dowel that was in the garage that I knew would come in handy one day (about 5mm diameter, 30 cm long) which I bent the last half centimetre 90 degrees to make a “hook”. I bent the other end into a rough “starting handle” shape to get some twisting-purchase.
Sitting in the boot, I poked the “device” between wheel-arch and seat-back. Located the bottom of the catch and twisted. I had tested it on the other seat first (the one that’s not broken) to get the idea.
Once the seat was released, I used long-nose pliers to reach into the seat to remove the piece of plastic rod that had broken off the handle.
I’ve replaced the head-rest (my main priority!). Now I’ll get myself down to the dealer and buy a new handle (as referred to on the post above).
Cheers for the advice.