Did the OP manage to get a copy?
Douglas Steele taught me at Chet's (some classroom music). He was delightful in a "Private Godfrey" sort of way. I remember him making an arrangement of "A fox went out" for our junior choir.
As I recall, at one time, he had been Beecham's assistant and his reminiscences contributed to the play "Beecham". He was also, I think, great friends with Humphrey Procter-Gregg.
In 1986 I was introduced to Arioso by its dedicatee, Gordon Thorne, and purchased my copy of Douglas' organ music shortly after. The edition was one of a series made by Forsyths in Manchester (I think of composers associated with that city).