thank you, all, that helps.
dd saw GP yesterday, he said to bring in a stool sample on Monday and then if it carried on they'd do malabsorption tests (meaning coeliac disease, I imagine - she was tested for CF at birth). He seemed surprised it was carrying on so long, if it was viral - plus she's had no temp, not seemed ill in herself apart from a streamy cold at the start of the week.
Today, we tried to change her formula in case it was lactose intolerance...gave her a soy based one, no diarrhoea: we were thrilled, but after her nap - ie several hours later - she burped, cried, then vomitted what seemed like her entire bottle plus undigested lunch
And now she wants nothing but water (been off solids for days anyway).
I rang NHS direct, who said to focus on rehydration and not panic...but have to confess, you lot are helping me not panic far better than NHS direct!
Any other thoughts or experiences very welcome, adn thanks to all who've posted.