Thankyou for the pictures and further information. I'm happy that my saddle is a Lycett. The elastics are secured to the rear frame using large hooks, as found on clothing, and looped through sturdy wire hoops at the front end. The individual cords are actually bound to the hooks using sailmaker's whipping twine. As the hooks and 6mm shock cord are readily available, I shall replace the rusty metal springs with shock cord.
FYI the length of 6mm cord required is about 2.5m, and hooks are UK Size 9, of which 14 are needed.
I have a handy saddle man who lives a few doors away from me, so I will get him to make a new cover for it after getting the frame re-finished.
Thanks once again to all who have commented!
Cheers for now
Doug