Dianna Wynne Jones fans! Can you name this book?

Question

I read it years ago, but I'd like to find it again (and can't remember the title).

It was about a group of step-brothers and sisters who discover a chemistry set one day that has all sorts of dodgy ingredients, including (I think) something that made people float on air and the philosopher's stone. They lived in a four-story house, one sister was called Guinevere (Gwinny), and they talked about being given too much change at the baker's at some stage.

Thanks in advance!

Answer 1

The Ogre Downstairs. I haven't read it, but the chemistry set's on the cover :)