Jim has it!
It's an Armstrong Whitworth Argosy - hence the cryptic clues in the intro - whistling and gardening = 'Whistling Wheelbarrow', one of it's more polite nicknames, and treasures sailing in = Argosy, or Argos, a treasure-laden ship.
This one is preserved at Cosford, and is the type I did my Para course on. The 'bomb aimer's' window, with escape hatch below, is supposed to be used for accurate 'spotting' of para drops, but was rarely, if ever, used.