Question Author
Kempie,concernig Menai, my original source was a Welshman called Dafydd (sounds Welsh enough to 'know'), who made that claim, complaining of the 'error'.My curiosity aroused, my later sources were found in old books, including Admiralty publications, which I traced online after googling the alternatives.
QM must be right, methinks. The correct form is whatever is in (forgive the wordplay) current use.
It's interesting that we say 'in dire straits' and not 'in a dire strait'. Either the comon form was always 'straits' or the plural is to emphasise the difficulty, it being more difficult to navigate a series of narrows (another plural: never 'a narrow' ) than just the one.