That is one of the parts that causes the problem yes
Firstly, thanks for your help with this, it is much appreciated!
Secondly it would appear from the pictures I have seen on e-bay and the like that the 'ball' is formed from a toothed plastic centre (which is all that is on mine) and a rubber piece that sits over this toothed section. (if this makes any sense).
Is this exactly how it works, a 2 piece part?
If this is the case it seems obvious to me that what has happened is the outer section has perished and falled of.
Cheers