OK - this is it! This is the definitive white wall cleaner.
White Magic microbre eraser. I bought a 27.5 x 11 x 4cm block for $5.00 from my cleaning supply shop or you can buy it from the distributor for more money -
http://www.whitemagic.biz
I cut off a 1 cm strip and cleaned each very yellow white wall in less than 30 seconds. The advantage is that the white rubber came up a true white rather than the off-white you get with a solvent.
The piece of eraser I used crumbled to...