Just to wrap this thread up.. It turns out that Moto Rapido still stock this part.
It's Rubber Buffer 19540021A and is just over a quid.
I have also learned that the "buffer" is 5.2mm x 5.2mm and that a 13/64" ball bearing will work if the correct part cannot be found.
Thanks to Paddy at Ducatipaddy
http://www.ducatipaddy.com/