That should be fine but lots of nurseries will charge a "consumables" fee so it may not actually be free. Ours is pretty high at £2.40 an hour but I have seen others paying less.
We send mine for 9 hours - we pay £7 for consumables and £10 for the extra hour but still pretty good value I feel.
It depends on how your nursery operates. Some do by the hour, some do half day blocks and some do full day only. Ours does things by the half day with a minimum attendance of 1 full day or 2 half days. They average out the hours across the year and their cost per hour (gov pays less than it costs per hour) so we get 11.5hrs a week funded at ours. A full day at our nursery is 8am-6pm, so we basically get 1 funded day and a smidge off day 2 (my boy goes in 2 days a week).
Ours does just the free hours if you don't want any others!
I send my little girl for 14.5 hours a week and we don't have to pay anything. Check with each nursery though as some of them only allow you take it in bigger blocks - full days as opposed to half days x