Hi Hamble,
I set one up from scratch at my kid's last school 7 years ago. It was lots of hard work and I think if I'd known at the start just what was involved I might not have bothered.
But I'm glad that I did. I got a real buzz from 'my club' and lots of satisfaction from having 'made it work'. I also made lots of friends through it and we had many a boozy committee meeting although this was when the club was up and running rather than in the fraught early days.
The Ofsted site goosey refers to will give you lots of info. I would also advise contacting your local council to see what they can do to help, both financially and for advice. In our area (Edinburgh) they didn't, at that time, give a huge ammount of financial help but let us use school premises free after the school day had finished. Contact other established clubs for help. Even things like looking at the club rules / constituion etc can give you ideas, both of things you want for your club and for things you don't. Get in touch with social work (Social Services in England, I think) as there are loads of regulations involving organisations working with children, including child protection issues, access for children with special needs etc, etc. Fortunately, we had a tame social worker on our management committee who helped a lot.
Probably the first thing to do would be to establish what the demand for an after school club (I'm sure it will be there) and also what other parents would be willing to help. In our case, the school newsletter invited interested parents to an initial meeting in the school and it took off from there.
It also helps to get the school and particularly the janitor / caretaker on your side.
If you have any specific questions you want to ask, fire away and I'll do my best to help. I'm away for the weekend but will look at this thread on Monday when I get back.