Many thanks Mr O..
I should have mentioned the = sign for the formula; yes, it was there.
I have never used the Fill command, being hardly aware of its existence.
However......
SUCCESS!
Fill > Series, select columns or rows, step value and final value, and it works a treat. Much better and easier than my old formula method from Lotus 123* days in 1983, or even its predecesor, if there was one.
Many thanks for the invaluable steer.
But I'm still puzzled about why copying the +1 formula into multiple cells has started to work so oddly, and it's going to niggle until I can see why.
*Or was it something like "Visicalc" then, before Supercalc? Memory grows dim.