Go into Millets or Blacks for some of the reproofing stuff they sell. I think it's Fabsil, but I suspect there are much better things now.

Or if you are a complete cheapskate, use Damp Start (not to be confused wuth Easy Start) It's spray-on plastic for waterproofing HT leads in your car, but it works a treat if you spray it along the dodgy bits. Don't overdo it, though.