I notice that a Wikipedia entry has appeared for Angus Stewart. I guess we all have novels on that mental list of 'I really should have read that and feel slightly guilty that I never have' and Angus Stewart's Sandel is definitely on mine. The hardcover first edition was published by Hutchinson in 1968 and there was a Panther paperback in 1971 but it has never been reprinted since. Consequently it has attained an astonishing price tag in either edition - although how fast it actually sells at those kinds of prices isn't quite clear to me.
Angus John Mackintosh Stewart was Australian by birth but moved to the UK following his father's academic career at the age of thirteen. Perhaps that was a particularly significant event for him since the protagonist in Sandel, (Antony Sandel) is also thirteen. Stewart never wrote another 'gay novel' and spent a significant portion of his life in North Africa experimenting with drugs and "self-discovery".