All institutions seem to vary as I recall when I applied to do mine so you need to check at the Uni of your choice, as you apply to the Uni direct and not via UCAS or any such similar system. At the uni where I teach p/t, the deadline for most post grad courses is 31 May to commence the course in Sept/Oct of the same year.
I should qualify the last sentence of my above answer by saying that the deadline for post grad courses in the particularly faculty I�m attached to is 31 May. Other faculties might have different deadlines.