Sounds like your white cable has snapped or the teeth have worn away so that it does not extend anymore.
Your best bet if you have a mk1 or 2 gs300 is to use TigerFish's guide linked above to dismantle your aerial and find out whats wrong first of all before you buy that replacement mast.
It is a very easy job to do, does not take too long, and also for the record I purchased the same ebay item linked last week and replaced my broken, grinding aerial!