Here's a wild idea, it might work I dont now. My wife makes dolls and one day she had some vinyl that was marked with pen (biro) she normally uses non acetone nail varnish remover to clean vinyl and it normaly work's, but this one wouldnt budge. We tried meths, turps, cif cleaner, all sorts. Then I sudgested trying a baby wipe, wapped round the bit with the mark, and wrapped with cling film to stop it drying out. We thought try anything. Any way next morning she removed the plastic and baby wipe, and the mark was gone, the wipe dissolved it drew it out and absorbed it over night. Baby wipes remove paint we discovered-handy to clean spills when your glossing. (worrying realy) might be worth a try on the marker stain. Or maybe get some rag (old cotton t shirt, or cotton wool) soaked in meths, wrap it round the area marked, and leave it a while?