I 'warm' more to the music of, say, Edmund Rubbra than I do to Simpson.
Edmund Rubbra and Robert Simpson are two very different composers. Simpson, in my opinion, was coming from a more Germanic/Austrian and Scandanavian influenced sound-world than an "English idiom" so to speak. Also the two composers obviously had two very different lives. I think Simpson truly wanted to say something different in his music and strove, sometimes to agonizing lengths, to get these thoughts on paper.
Whether Simpson will be remembered 20 or 30 years from now is hard to say, but I think his music deserves to be heard as he was daring enough to still work in a tonal idiom and it's this kind of persistence that I really admire about him.
I just bought his box of symphonies on Hyperion, because I felt that the man truly deserves a thorough listen and I'm sure there's going to be a lot music that I'm not going to care for, but hopefully Simpson will surprise me. But Rubbra, Simpson, Arnold, Alwyn, Tippett, Britten, among others were doing something very different in British music during their lives, which is always refreshing.