Well it hasn't happened yet, but I've taken the first step, buying the dummy cover that fits over your existing perished antenna without having to cut all the rubber off first.
You clean up the rubber, and glue this cover over the top. I got it from the Partsforsaabs Ebay page, because it was cheaper than getting it on their website and paying the postage!! And I have to say, it fits very snuggly and securely. Those guys have done a good job.
Now I need to find a suitable glue......