Along with his more academic work, he has a successful career as a commercial composer and wrote music for both television shows and commercials. One of his more famous claims to fame is that he wrote the soundtrack on BBC's The Hitch Hiker's Guide to the Galaxy (1981).
He focused more on the use of brass in his pieces starting in the 1980's. Souster wrote electronic based music for a variety of different ensembles including solo trumpet, solo flugelhorn, and brass quintet. Souster even wrote the first piece for brass band and electronics titled, Echoes (1990).
Although he wrote a large amount of works for brass and electronics, many of the recordings are difficult to track down. I was able to find one recording of his brass quintet piece, Equalisation for Brass Quintet and Live Electronics (1980), which can be found here.
In Equalisation, Souster mainly used two electronic devices. He applied a digital delay line to alter the acoustics of the room and a pitch transposer to add parallel intervals to the live music be played. If you would like to learn more about the piece itself, detailed information provided by the composer can be found here.