Checkmate wrote:...is it possible for HO to accept an application on Form SET (M) if a British Spouse's passport has expired
i.e can an expired passport be submitted to prove that the spouse is british - or must it be a valid/current passport?
The SET(M) is for the spouse of someone settled in the UK, it means the person settled in the UK may not even be British. And lots of Brits don't bother getting a passport anyway, so the answer to your question is yes.
But to play it by the book, you would submit your long birth certificate and something that shows you've been settled here for the last 3 years - like a driver's license, tax return, etc.
And to play it by best industry practices, your cover letter would explain any unusual circumstances why your passport was not renewed...