I think I've found the truth about the elusive Mr Hannan, but none of the original information I was given was correct. JOHN JAMES HANNAN, aged 24, the son of George and Jane Hannan of Coatbridge, died on 20/07/1945. He was serving with the Royal Naval Patrol Service aboard the HMT Nebula, a minesweeper. His record says Died On War Service. Apparently unable to swim, he drowned in an accident and is buried in the British cemetery in Recife, Brazil.
It seems to me that the tale I was told was a wildly inaccurate version of this story.
Thanks to everybody on Rootschat who contributed.