I have just come inside from the shed and Mick was fairly close to the mark.
The worm shaft runs inside brass bushes at each end. These bushes are slightly smaller than the hole in the housing that they fit into to and the space is padded out with O rings. This horrid arrangement allows the worm shaft to move sideways under tension, compressing the side of the O ring
The fix would be to make new bushes either from brass, or replace the O rings with hard nylon or similar material rings, both of which are beyond me considering that I dont have a lathe
This site here explains it. and the pictures tell the story.