All fixed. The washer had indeed disintegrated. I replaced the whole fitting, in the end, a slightly different design but it is working fine.
I also worked out why the pipes were slightly out of alignment. Someone (not me, of course) must have crashed the boat into a lock gate and the tank had shuffled forward. I will need to find a way to stop it moving again.
Meanwhile, the domestic water pump decided to stop working. I disconnected it from the plumbing, unscrewed some of the screws holding the two halves together, and it decided to start working again. Aren't boats fun.