Thanks Simon, what's the goof off equivalent in the UK? There seem to be a few products available but it's not clear to me what would do the job. I tried with IPA and meths (on the spots that will be hidden) without any noticeable improvement. I fear I'm in danger of making things worse.
Sharpie is definitely a possibility, I've used them before to hide the odd scratch in furniture and even to cover bleach strains on a jumper :-)