I was told by a lady who used to clean up toys for charity shops that you could use a cosmetic deep cleansing lotion to do this - just the stuff for teenage skin care by Clearasil or something like that from Boots or Superdrug. Haven't tried it but maybe you could dab a bit on the edge with a cotton bud and see if it comes off. Failing that, a paste of bicarbonate of soda or a dab of white vinegar seems to remove just about anything from anything! There are also stain removers called Stain Devils and they do one for ink. Hope something works!
I got a white sponge free with a detergent a short while ago, called Erase Away. If you can find it in any of the supermarkets, I swear by it. It removes marks from trainers, tea stains in cups/mugs etc. and just about anything I've used it for. Just add water and "erase away". Unfortunately as mine is still useful, I have not had the need to seek another out so cannot tell you exactly where to get it! Good luck hunting and then erasing!
Methylated spirits will remove biro marks so that might be worth a try.
I think sportster05 is talking about a magic sponge by JML which is available in Focus, Woolworths,Asda and other JML outlets.