Found the info:
EXTRA FINE NIB PERMANENT WHITE PILOT PAINT PEN MARKER [ASIN: B00203ZHIU]
Bought from Quickdraw stationery UK via Amazon marketplace.
Or here for £2.92 delivered:
http://www.amazon.co.uk/gp/aw/d/B00203ZHIU
Take your time though and definitely practice on an old tyre first.....if you press too hard the ink will flow too fast and go places you don't want it to.
I also used this to do the CAT lettering on my XLS front bumper.
Dan