It's the bellows part of the mechanical seal which went. Looks like crud (the stuff you scrape off a used immersion element) got stuck between it and the shaft. Although there's no differential movement between the two, I think it must have been responsible in some way.
I tried buying a cheap seal off ebay for three quid, whilst it fitted the shaft ok it was too long for the housing. Had to spend a whopping £9.20 on the correct seal which should be here tomorrow, if the supplier is to be believed.
I'll finish the repair, taking additional photos and add it to the Leo page, once I've tested it.
