Six weeks on, I have made more progress and have a potential breakthough. Can I ask the opinion of anyone following this thread?
My William Starling's father John might be Starling-1020, a John Starling from Isleham who was described as a "waterman" in an Isleham christening record of 1816 but then "labourer" in four subsequent records from 1819 to 1829.
That matches to my William Starling - the son of Starling-613 - who was born "on the sea" and whose father was described as a "labourer" on the 1836 marriage record in Worlington, Suffolk.
Worlington is 5 miles downstream from Isleham on the River Lark, 20 miles upstream of Ten Mile Bank in Hilgay where William later lived with his second wife.