"It makes me wonder who would be the market/audience for such productions in Britain c1956/7 "
There seems to have been an on-going interest in shanties throughout the first half of the 20th century.
Remembering Hugill's pointing out the international nature of shanties, the Library of Congress was recording sea songs and shanties from New York, Wisconsin, Virginia and California as early as 1939.
Whall published in 1910 and again in 1920, Colcord in 1925 and Doerflinger in 1951.
Stanley Slade was first recorded in 1943 and it was thought there to be enough of an audience for HMV to issue his singing commercially.
Incidentally, the late Tom Munnelly wrote a fascinating long article on Irish sea songs which contains a very evocative description of on of the great blind storytellers, Henry Blake, hearing the crew of a sailing ship sailing out of the Shannon estuary singing shanties - I'm pretty sure Tom said that none were collected in Ireland.
Jim Carroll