Dropinthe - your nursery will have to have filled in development charts (whether it's private or not - I think most day nurseries are private aren't they?) like Foxinsocks describes. If you want to see them just ask, although at my Ds's nursery they had parents' afternoons (not very often) where they went through them with us. These are then sent on to the school and form part of their continuing records.
Re starting him at preschool 6 months before - I don't see why not, you can start when you like usually. I started DS1 at the school pre-school in Jan 04, and he started school in Jan 05. I would have done it later, but I was having a baby in April and wanted him to get settled in before then. Like I say he stayed at Day Nursery too.
If the pre-school isn't attached to the school, I would say don't bother. Your child probably gets all the stimulation he needs from nursery. The only reason I took him to preschool too was to get to know the kids and the school. The pre-school kids had extra visits to the school reception class in the tern before they started school, and they went to see the KS1 christmas play and so on, so by the time they started school they already knew the building, the teachers and most of their classmates.
It doesn't always matter, some kids will get on whatever they do and can get by without being 'part of the crowd', but to my DS1, it was a very important part of his personality - his friendships are more important to him than his family, I think! And yes, I did notice a difference in settling in between the preschool kids and the ones who hadn't been, sorry.
However, like someone just said earlier (sorry can't remember who), by the first half term it's all evened out anyway.