The finished quilt
It's summer. My sewing room has lots of windows and catches the sun in the afternoon. On a warm day it gets hot - on a hot day it's like an oven. So mystery solved as to why the chalk didn't dust off or wash out.
I remember marking this top and starting the quilting before going interstate for a week, then another week or so before I got back to finishing the quilting. Meanwhile, the quilt has been sitting, folded in a sometimes very hot room for weeks. The chalk had heat set.
I received a lot of advise on how to remove the marks, thank you all. I trialled a couple of products on fabric scraps from the quilt and settled on Sard's Wonder Stick. The samples were chalked really heavily and set with a hot iron. Sard's completely removed the marks very quickly in cold water - so I went ahead and applied it to the chalk on the quilt and washed it in the machine.
There are still some orange markings I can see on the white fabric. I was fairly sparing with the product, so another go should hopefully remove the rest of it. It's a busy, blended quilt top, so unless you're looking pretty closely, the chalk won't be noticeable.
After Sard's Stain Remover
Lesson learned folks - mark and quilt quickly (I normally try to do it within a week) don't leave marked quilts sitting around in warm places, and that includes cats sleeping on them!