Thanks all. Robin Blain rang me today to say that Sunbirds used to make these as standard, and can still get them made. Since I think my son-in-law will make his own, your ideas and suggestions are much appreciated and show the power of the site.
Ketil, you are right. I dowloaded magazine 18 as per Arne's advice and pages 20-23 make it all clear. What a memory mine Arne is.
Peter, thanks for that offer; I've passed it on. Yes, send a pic please, thanks. Why do you need Sea-wock as well as your other boat, whose name escapes me at the moment? I remember seeing her when she was for sale several years ago. A good buy I should think, and with an internal diesel I believe.
BTW I've encourages my daughter to join the JRA. I paid for her first year's membership but I'm not paying again :-)