Thanks. This morning it seems to have cooperated. Hopefully there will be no more issue. I shall see.
I am a little unclear about the what and why though. First "Settings>Face ID & Passcode>Enter a passcode" - when I got to Face ID & Passcode I see a page of things starting with USE FACE ID FOR with four things - iPhone Unlock, etc. and Other Apps 7 apps. I didn't/don't actually see "Enter a passcode". I was seeing that all four of those switches were off for some reason - iPhone Unlock, etc. Last night when I tried clicking iPhone Unlock it jumped into a process of looking at my face. It was different from what I got this morning just now.
This morning it came up with my face in a circle that rotated around for a little bit with me moving and marks building up around the circle. That seems to have worked this morning, and when it completed all four switches were on and all seems normal.
Last night rather than a circle the camera image looking at my face filled the screen area, with four rounded corners marked within it. It kept saying to move, face not in, move lower, etc. It eventually failed each time I tried. As I said this morning it seemed to go as you all say - easy, just automatically using the circle.
he mysteries, maybe for another day, are why did it need resetting at all, and what was going on with the process. I can only note that it was night and the rooms I tried in were pretty dark so what it was seeing may just not have been satisfactory.
Anyway, thanks.