I finally got to talk to Bradtech yesterday and they were able to clear up the problem.
The correct pipe-tail to connect to the toilet is plastic with an internal conical section which goes up inside the water valve fitting. The locking nut tightens up on the outside of the fitting as normal.
I now understand that the brass fitting employed by the original boat builder/fitter was never going to work, trying to make the seal by butting a square brass edge up against a sharp edge section on the nylon water valve. That's why they employed lots of silicon to bodge the resultant weeping joint, having stripped the thread on the fitting.
I can already see that this new pipe tail (supplied gratis by Bradtech) is going to sort my problem.
Whilst talking of Bradtech I cannot praise them enough. They seem to have a comprehensive range of Thetford spares (unlike Thetford themselves!), they are very competitive price-wise, and my goods arrived by courier within 24hours of my last 'phone call. Their tech-support department (Bill!) is very knowledgeable and friendly. Altogether a pleasure to deal with.
Thanks to all who contributed.
Guess what I'm doing this weekend?
David