In reply to Bloodfire:
Fine tip (0.6mm) Staedtler Lumocolor permanent pens
http://www.staedtler.com/en/products/markers-refill-ink/universal-pens/lumo... are easily the best option of anything I've used over the last 20+ years. Although they aren't cheap and I always seem to fail in stopping students from 'borrowing' them.
They work great on all the variations of OS laminiated maps and can easily be removed with acetone (i.e. cheap nail polish remover from the pound shop) - PROVIDED you don't wait weeks before removing it. The black is perhaps slightly 'more' permanent than the other colours but is still removable.
That said, most varieties of permanent marker work fairly well if you can remove the markings within the day but equally, you'll struggle to get any of them off again if you leave it over a month. The biggest difference I've found is that compared to other makes (especially cheap ones) the fine tip Staedtler are just much better to write with, don't smudge, don't leak, don't dry out and will last for ages provided you can hang onto them that long...
Post edited at 05:49