Well, was. It's all sorted now, thanks to a handy mate of mine.

The plunger didn't do it, and neither did the tried and tested "blow down the pipe" method. I didn't want to tip a pile of chemicals down there to melt it out as it goes directly into the cut. In the end it was the "get a big thin stick, go outside the boat and poke it up the hole" method the came up trumps and cleared the blockage. I've learned my lesson (no more coffee grinds down the sink), and I've learned that little bit more about boat plumbing. Sink is connected to a pipe, pipe is connected to a hole in the side of the boat. Can't get simpler than that really.

Now I need to work out how to stop my hair going down the plug hole in the bathroom. I'm not looking forward to sorting that one out. How do you even get to that pipe anyway?