if linked to primary school it's usually either upto 5 mornings or upto 5 afternoons.
you also get groups based in places like church halls which will be whenever they schedule them, usually mornings, sometimes just 3 times a week, sometimes 4 or 5.
then you have private day nurseries where they basically knock off the funding from your total bill. you can put sessions together though, so you could have 6 hours monday, 6 hours tuesday and 3 hours wednesday for example. anything beyond this you pay for. and of course out of term time you pay in full.
Your ds will only qualify for the funding from next September(term after turning 3). Up to 5 funded sessions which may be 2.5 hours or possibly 3 depending on whether your LEA has increased it by then. So even for an attached school based one it may only be part-time but you have the choice of using it at privately run preschools/nurseries or childminders who are registered instead. Primary school allocations at Reception age are not linked to Nursery attendance so there should be no advantage in that respect.
He'd start Reception during hte academic year starting Sept 2010,at 4 going on 5, for which you'd possibly apply as early as around his time next year. Nursery places are on a similar timeframe a year sooner so check your LEA website. The Children's Centre may simply claim the funding when he becomes eligible ,in which case no need to move if he is happy there.