With the amount of plastic and rubber on a Mk2, I've tried loads of things. The best solution for me (mainly because I use autoglym SRP as my usual polish) is autoglym's bumper care, as its specially made to remove the SRP stains. A pencil rubber is supposed to be good, and I've also used a toothbrush and a little WD40 (although this removes wax too so be careful).
Peanut butter didn't really work for me.