Thanks for all the replies so far and keep them coming! At the minute I'm heading for Specialised Hotrock 20. There doesn't seem to be anything better spec'd on a 20" wheel. Looks like I'll have to wait til she's about 9 before investing in 24" wheel a bike that's going to last her more that 2-3 years
Specific responses - on your suggestion on keeping rigid, sam - yeah I'd steer completely clear of a full sus bike - kids bikes weigh a ton as it is and at the price bracket you're looking at for 20" wheel bikes, the suspensions gong to be so crap anyway. And I wouldn't fuss about front suspension for the same reason - but as with adult MTBs, as soon as you get out of total bargain basement land you don't seem to be able to avoid front suspension.
Thanks martin - your recommendation helps me head towards the spesh. On being a bit heavy - has anyone found a kids bike that isn't? Why does my daughters 16" wheel schwinn weigh about as much as my £250 knock about ATB? Obviously someone half the wight isn't going to have a bike half the weight because there's going to be the same range and weight of components - but surely manufacturers could take a lot of material out of the frame to support 3-4 stones instead of 10-15?
Had a look at Evans thanks Mike - best range of kids bikes I've seen on the web so its great for comparison.
And thanks Dom - yeah for knocking about, one of the important functions of bikes for children, a BMX is going to be perfect - but I want to drag my daughter out for rides down the road and off it, and living in the Yorkshire Dales, you can't get far without some gears.