I Googled the above and someone suggested brake cleaning fluid because it evaporates and does not leave a residue. Thinking brake cleaning fluid may be a tad harsh on the surrouning paint work and decals I compromised with WD40 and a stiff bristled tooth brush. It worked a treat getting rid of most of the black grime. The toothbrush I used was a special denture brush which has longer bristles. (I dont go to the dentist any more I just put my teeth in the post) I also use these for cleaning around the various nooks and crannies in the kitchen at home. The WD40 can was the one with the new lift up tube system which is much better than the old red tube that you bunged in the cap.
In the furure I may try Swarfega as this too should lift all the dirt out and be less runny.
In the furure I may try Swarfega as this too should lift all the dirt out and be less runny.