I use a very long feather duster, which if used regularly is sufficient. If they haven't been cleaned for a long time though you may have a thicker build up of dust - when i moved into my new home i had this problem so unscrewed the long handle off my mop and attached some old rags round it. I used this along with detergent and hot water. Depending on the space between the radiator and the wall you should be able to do this, if not i would go hunting for something long, strong and thin. Good luck ;-)