Hello there
Saw you are moving to Hitchin. We live in Letchworth (the next door town).
As cat said, the terminology is confusing. If you are only looking for 3ish hour sessions at a time (ie not full-day care) then round us there are (1) pre-schools (usually independent of schools) which tend to take children from 2 or 2.5 and will keep them until they start reception ie the Sept before their 5th birthday (but some children move sooner) or (2) nursery schools/classes attached to primary schools, which will take children in the academic year in which they turn 4 ie from about 3.5.
My DS is just turned three, so may be an academic year ahead of yours? Or maybe not, if your DS is an Aug birthday. My DS started preschool two mornings a week in Jan, but could have started at just turned 2. We currently pay for this, but from next term he will get this paid for as part of his 15 hours (though I think he may still only go 2 mornings).
Then in Sept (or Jan if he were a March or later birthday) he could start in a nursery class attached to a primary. This would have to be 5 mornings (or 5 afternoons) a week. Then in the following Sept (ie 2012 for him) he will start in the reception class at the primary, full time.
However, I could choose to keep him at preschool until Sept 2012, which has the advantage of being more flexible in that you can choose the day they attend and do a mixture of full and half days.
I find it very confusing myself!!
There are also day care nurseries which primarily cater for working parents and take children 0 - 5. You can use your 15 hours at one of these too.
If your DS was born before 31 Aug 2008 then definitely look at this now www.hertsdirect.org/comdirectory/childservdir/parentcarer/childcare/nurseryadmissions/ - this is the stuff about applying for school-based nursery places for this coming academic year. Even if your DS won't be eligible until the following academic year, it might be useful to see how the system works.
Very happy to try to answer any more questions, if I can. Good luck!