It definitely depends on what the LA will pay, in West sussex it is £3.77 per hour for the standard 15 hours or part of over 38 weeks, but if your child attends a nursery who spread it over 50 weeks then the amount is pro rartered down as there is a top annual amount paid.
Your Local Authority should have a children's service/line which will tell you. I did look for Devon but their website is down! Our LA isn't keen on posting the info online for some reason, I guess because it is a bit complicated depending on hours your child goes to nursery etc.
Incidently, I am in London and our nursery has just written to us, they are sorting it all out for us and we aren't being asked to top up the hours that are free. We are saving £183 a month which is great.
The nursery aren't allowed to top up the funding. I do cynically wonder if the LAs are cagey about giving details of their funding so that parents won't realise that it's the nurseries and childminders themselves that are being forced to make up the shortfall. I'm not even allowed to tell the parents who claim how much of their entitlement is actual LA funding.
But actually I guess they are topping it up. I didn't know that. I wouldn't have expected that personally. If the funding was x, then I'd expected to pay any on top if x wasn't = an hour at the nursery/childminder.