aha, managed to get the snapped off bit in my aerial out enough to be able to screw in a new one... its a 5 mmm thread, so basically attacked it with a 4mm drill bit to drill out the core, then it just kind of crumbles away and you can screw a new aerial in.
I basically thought it cant be any worse than it already is, so might as well take a drill to it and see what happens.
and then when screwing in the new aerial, have applied a dab of copper grease to hopefully stop it rusting up again, but well happy now, new rdx stubby is on!