hi frederickwilliams, i'm sorry but i don't think we have any way of helping you there with an account recovery procedure for a third-party service unfortunately...

Dear Philipp, I do understand: part of the reason for my distress was the apparent complete lack of any helpline or other form of assistance provided by Google. However, I think (and hope) that I have inadvertently stumbled across a solution, though I'm painfully aware that I don't know how I did it, and could not repeat it if the need arose again. I did note however that Google in its advice seems to use the word"passport" in a whole range of meanings, and fails to be specific about which sense it is being used in a particular case.