For those that are interested, I got one pair of springs from Land Rover Orphanage and a set of three different pairs from David Welch Motorsport. None of which were correct but allowed fiddling! Using the tried and tested method of 'suck it and see' I've got a medium and a strong from the D W set fitted and things seem reasonably happy. I've even got the feeling that my mid-range flat spot may have significantly shrunk but I'll have to experiment further ...