I have had some success using Magic Eraser - dry removing pencil.
Peckish - what was the reason why Alex Anderson does not recommend Frixion?
I was going to say for a "next time I do this" - to use the Frixion because one of the things that I love about it is if I change my mind when marking a quilt, I take the heat from the iron and it disappears so that I can change my mind.