I cant add much really.
Here a place for a child over 3 is called 'pre-school' and nursery refers to the under 3's.
Every area is different though, so may differ in hours (although I believe most if not all places are 15 hours a week starting 2010) but these may be split differently depending on the nursery/council.
So for instance at my private nursery they currently say that a session is 2.5 hours (that may rise to 5 sessions of 3 hours, or it may change to 6 sessions of 2.5 - my nursery havent been told yet!) This has to be spread over at least 3 days - again that won't apply everywhere.
AS YKNotC says - you need to visit the nurseries yourself to find out if they have funded places available - then apply. They usually do the paperwork, i.e. you wont actually 'see' a voucher. They may not have enough sessions free, in which case it may be possible to use two nurseries. But imo its better for the child to go to one venue especially if they arent used to being away from you!
Some nurseries are only open term time and others all year, and the hours are also dependant on the nurseries.
The vouchers are only for term time ie 38 weeks a year, however my nursery is open 50 weeks a year and they spread the available 'money' so the parents dont get high bills in the months there is no funding.
So
start looking now
Try here
to get yourself a local list of places.
Have a look at ofsted reports or find a local forum and ask the parents!
If you find some places you like, then ring them and tell them you are looking for funded pre-school place for your 3 year old and ask if there are places available.
They should help you from there!