Thanks Brion
I realsie now that getting a copy of your book would have been a wise move. The re-rigging has been more involved that first thought, but that is from the perspective of ignorance rather than experience.
I have taken the advice and ordered a copy of the "Riggers Apprentice".
The rig is masthead, not fractional.
The puzzel for my thinking is that triangular plate that the caps and uppers attach to, then it in turn has a pin attaching it to a chainplate. With one heavy wire, the cap at 5/16, it seems strange to have the upper sharing it when it is only 1/4". I had a look around tha bay where we moor and there is not another rig like it.
I have put some photos on this page that show the "bridle plate" for want of accurate riggers jargon.
http://adams35.multiply.com/photos/album/2