Hurray, we have hot water again. The Morco is fixed and the Eberspacher decided to work properly when I tried it this morning. I think the split union in the cold water inlet to the Morco was probably the result of ice. This exceptionally cold weather really has tested the systems, especially since the boat has been left cold for 6 weeks or so.
I realise now that draining down the Morco deals with its contents and the outlet, but not the inlet. The only way I can drain the inlet, which is four feet above the floor, will be to drain the cold supply at floor level, so tomorrow I'm going to rig up something which will do that. That should protect the water pump too.