I removed the prop shaft from the tube, fitted a length of silcone tube over the end, and blew into it. Result...a stream of bubbles coming from where the rudder linkage wire exits the (new) rubber bellows. I'm guessing that the wire is too thin, so it should just be a case of swapping it for a thicker length.
****************************************(ten minutes later)********************************************
I think (think) I've cured the leak. I now have a slightly larger diameter push rod, and I've filled the bellows with good old Vaseline, and cable tied the exit point as well.
It's been back in the bath, and good lungful (or the best I can do) didn't produce a single bubble, so I have crossed fingers that all is well now. It's still in the bath, weighed down to keep it underwater...I'll give it an hour and see what's happened.