My encounter with him occurred at Petworth House in the early Seventies. My wife and I spotted him gazing earnestly at a Fuseli painting. Being a fellow Cornishman, I introduced myself. He launched (I swear I saw a bow wave) into a very loud and lengthy lecture on the wonders of Fuseli, interspersed with naughty snippets about Lord Egremont.
Within five minutes the fairly crowded room was emptied; even my wife disloyally snuck off. I was left stranded with him for a whole hour. I still have not quite recovered.
Brighton, East Sussex