I have tried remedial action that did not work. However, I have just rung Buy Spares (I have only recently found your site) who have identified the parts and said that it is available (although as a special order that it could take up to 28 days). However, I have now discovered that the metal 'lug' or protruding strip through which the retaining bolt secures the concrete block to the drum has sheared. Mmmm. I think this means a welding repair but I don't know whether this is feasible.
Anyway thanks for the advice.