Just a whisker after the 1930s, since you ask! Hope that helps!
Cor! Must be Geoffrey Burgon!
Sadly, Geoffrey Burgon died in 2010. So....not him.
I am not terribly knowledgeable about modern British composers who are not symphonists. As I am deducing that this may be a composer whose idiom became much more "advanced" as he got older I doubt very much whether I shall be able to identify him.......certainly without more clues ::)
Well, put it this way - not only did this composer write no more symphonies, he also wrote nothing more in B flat (at least as far as I know)...