Andy,, id suggest powerwashing them while on the car give them a scrub with a wheel brush.. easily available from halfords, rinse off again then take wheel at a time and powerhose inside. you can get products from autosmart cheap enough like tardis or in extreme cases they do one called fallout u might want to look at one thing i woud say if you goto the likes of autosmart or autobrite ensure you get acid free wheel cleaner .. the most popular wheel cleaner i find is billberry wheel cleaner, its a simple spray on scrub around and rinse off. once fully clean the best wheel wax in my opinion is swissvax autobahn but i have heard of people having good results from hard collinite waxes.. once the wheels are cleaned thoroughly you will probably want to clay them then rinse them again to ensure free of contaminants before waxing..