go onto detailingworld and look for a company called autobrite (i think)...they do a water based cleaner with a foam spray bottle for £20, its called 'very cherry wheel cleaner' and comes in a 5ltr bottle..... you dilute it to say 1:20 or 1:30 and your away
spray it on two wheels at a time, leave for 2-5 mins depending how hot the weather is, then jet wash off, and hey presto clean wheels and you dont have to touch them once!! may require 2nd application for heavy dirt.... i cant recommend this stuff anough and it has to be used to be believed. Only thing id say is that you have to use a pressure washer or else it wont work (not necessarily on the highest pressure setting)
much better than acid or alkaline based products and less likely to erode your wheels
oh and the misus is picking up a laser blue cooper S with chilli pack and white wheels on Friday from sheffield Mini...looks awesome
Tim