Thanks again for your feedback Mikey
At least with php, when it works, it works !
I tried your suggestion with the min-width, but no joy.
Just for fun, I created a new empty page, reset the styles, and added media queries to the css.
The page simply changes background colour and width when the 'threshold' is reached.
Looks like it happens when body width is 854 pixels again
I must be missing something fundamental, time for a glass of something.