I got some of the gutta off with a stiff bristle brush. The rest of it came off easily by scraping it with the edge of a credit card, after I had heat-set it and then soaked it in water overnight.
Thanks, I posted it as a new discussion on the group page. I already have pebeo setacolour paints and will try this project with those. They may not be as vibrant as the ones Lea uses, so I'll have to see.