[Arran]

 I cleaned up the cloth on a Canadian Marconi portable but trying a bunch of things, in it's case it had black greasy dirt like dried up motor oil all over the top. First I used some gojo hand cleaner, but I think I also used mineral spirits, alcohol, TSP, and finally lacquer thinner. Eventually I painted over the cloth with Watco lacquer to restore the doped look. I would start with Gojo and see where that takes you, but carpet or fabric cleaner may help if that doesn't work. Another option if it's really bad is brake cleaner, which is perchloroethylene in a can, the same stuff used at the drycleaners. Keep in mind that this cloth is doped, meaning it's clear coated with a varnish or lacquer, so if you go too crazy with chemicals you may have to recoat it.
Regards
Arran