Apologies for the hiatus, I only just got round to finishing the job. I had a few issues, not least of which was that the centre bracket bar I bought from eBay was bent (from a car which had a heavy side impact, I guess). Big vice, blowtorch, jack and big hammer eventually took care of the problem.
Anyway, I effed about with the U-brackets, lower swivel brackets etc. for ages, including cutting out part of the plastic cover over them, when I suddenly saw the solution...
DISCLAIMER: I KNOW NOTHING AND NOBODY SHOULD EVER FOLLOW THIS COURSE OF ACTION OR YOU WILL DIE HORRIBLY!!!
Having removed the U-brackets and stared at the resulting gap whilst ineffectually waving the seat-backs up and down and muttering obscenities under my breath, I noticed that the front bolt-hole for securing the U-brackets was exactly aligned with the snecks on the seat-backs! I cut the heads off a couple of long (3-4") M10x1.25 bolts, ran locking nuts up the threads, screwed the headless bolts into the aforementioned front U-bracket bolt-hole and tightened the lock-nuts.
VOILA! It works and now I have split rear seats with headrests.
Thanks for all the advice, pointers and help. Much appreciated chaps.

Anyway, I effed about with the U-brackets, lower swivel brackets etc. for ages, including cutting out part of the plastic cover over them, when I suddenly saw the solution...
DISCLAIMER: I KNOW NOTHING AND NOBODY SHOULD EVER FOLLOW THIS COURSE OF ACTION OR YOU WILL DIE HORRIBLY!!!
Having removed the U-brackets and stared at the resulting gap whilst ineffectually waving the seat-backs up and down and muttering obscenities under my breath, I noticed that the front bolt-hole for securing the U-brackets was exactly aligned with the snecks on the seat-backs! I cut the heads off a couple of long (3-4") M10x1.25 bolts, ran locking nuts up the threads, screwed the headless bolts into the aforementioned front U-bracket bolt-hole and tightened the lock-nuts.
VOILA! It works and now I have split rear seats with headrests.
Thanks for all the advice, pointers and help. Much appreciated chaps.