Garry, I can't add any more to the discussion of where the "backdoors" line originates, but I do know that it's a relatively new addition on Martin's part. I have a recording of a gig I attended in 1988 when Carthy & Swarb first reformed their partnership and he sings the Penguin "wild water and bitter sky" line here. I've seen the duo on numerous occasions in the intervening years and haven't heard them sing this particular song since (and I've checked my various recordings), until the "Straws In The Wind" CD/tour in 2006 when Martin sings the "backdoors" line...

Not sure what this proves.

Probably nothing!