to me the only DS members that really made a significant difference in the band-sound apart from Mark were Pick Withers ( that groovy, laidback, easy feel is such a big part of the first records and never again matched) and I have to say; Alan Clark. I feel his playing style is such a big part that made the Love over Gold record, and also in the live shows after that his playing is a signature part of the band-sound.
I do find it a bit strange that Pick never really appeared in another professional outfit after DS, I can image a lot of other artist would want him in his band after him leaving DS, being such a great drummer.