Pipercross sell kits fo cleaning and re oiling the filters. Most places that sell the filters also sell the kits and cleaners oils. The oil is about £4 - £5 a tin.
Thanks guys, got a can of K&N spray filter oil for the Chim but instructions say "spray along the pleats" ? Thought I'd check before spraying it on the sponge and seeing it melt and turn into a gooey mess !