I decided to have a go at fixing the prop shaft coupling myself. With a little help from folks at the Canalworld Discussion Forum I found out what needed doing. I dug out my trusty torque wrench and set about undoing all the bolts that held the propshaft in the coupling. With a little persuasion the collet came out of the coupling and I could then slide the shaft back in with it and tighten everything up again. Job done. About an hour's work.
The coupling is the big round thing at the bottom of the picture. The eight bolts at the top end hold the shaft in. It's not very accessible because of the engine's soundproof cocoon but for once the cocoon made life easier because it was comfortable to lie on top of while I worked.
While I was at the boat I thought I'd check under the bed as there has been an unexplained coolant leak from the engine for the last couple of weeks. The puddle under the calorifier said it all. Looks like the boat's still not going anywhere until I've found some PTFE tape and clamps and remade one of the joints where the engine cooling system joins the calorifier.