there's bound to be cancellations, some will probably be at very short notice, as people approach the test date and realise they aren't ready, make sure you know the days / times your instructor is free, although a lot will shift around other lessons if you get a short notice cancellation.
its just a case of checking daily / multiple times a day and hoping for the best, but one will more thank likely come up, if your test centre is that chocablock until January, they will likely draft in another examiner from elsewhere, which should make an additional 6 - 8 test slots for each day.
also there will be people in the same position as you, with earlier dates who are waiting for cancellations, so if they snap up an earlier one before you, it'll leave a their one free.
if you haven't rebooked the test, don't hold off, just book the late one and keep looking for cancellations, if worst come to worse and there's no cancellations (unlikely) at least you've started the 8 or so week wait ticking.