On the Halfords site they're sized by age but I don't know if my DD has the right length legs for her age! Also we want it to be a surprise for her birthday so don't really want to take her into the shop to try them out.
Is there a rule about bike size/inside leg measurement that I could work from?
I'd say floor. We got a bike for Dd a couple of years ago, she was 9. She has very short legs for her age, when we went to the bike shop we were advised to get her a 24" wheel. I was but the owner said to get a low frame one, but with the bigger wheels, she has got 2 years out of it so far.