Phew! It's bone dry and the mark has completely gone thanks to Vanish (bless whoever invented that stuff - it's my go-to for all stains)) and a short warm wash.
I'll post a review of the Evergreen soon as I don't see many versions around other than the pattern testers'.
Lessons learned:
1. Never use fabric markers on the right side of garment fabric - at least not without first testing on scraps. I should know that by now
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2. Someone on the forum will probably know.
@Acorn 's initial reply set me on the right road. (And thanks to all who posted for your support)
3. Don't panic!
Happy ending
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