Catalogued as ‘English School’, the 3ft 3in x 2ft 6in (99 x 75cm) oil on canvas was initialled JR 1949.
It came with an old label for one of South Africa’s oldest commercial art galleries, the Pieter Wenning Gallery in Johannesburg (now the Everard Read Gallery).
The oil was eventually knocked down to one of six phones active on the lot for £5800.
The portrait had come from a local dealer, who had “attached no great value to it”, said Charterhouse’s Richard Bromell.
“Thankfully the power of the internet found bidders who knew more about the picture than we do, and it sold to a London dealer who specialises in 20th century art.”