From what I remember he had to remodel his action in the mid eighties due to back injuries. He was very front on and a geniune quick bowler until then. From about 86 he dropped down to fast medium and became a swing bowler.
Diley now would've play alot more test matches because he wouldn't have been playing 15-20 county games as well, that killed alot of decent bowlers in the 80s/90s. Poor Management didn't kick in until the 90s, simply because there wasn't any in the 80's. They pretty much got picked/dropped via the radio/tv on Sunday, reported for duty Monday or Tuesday (depending on county games finishing), in whatever condition they were and played Thursday.
Norman Cowans was another who dropped pace to manage workload.