johnwmcc on "Twenty Fourteen - mobile site compressed to approx 1/3 screen width"

Brilliant - thank you so much. That's fixed it. I moved the menu settings in the Child theme to apply only to screen width over 1025px, like this:

/* Move main menu bar container but only for desktop width browser*/
@media screen and (min-width:1025px) {

  #primary-navigation {
	position: relative;
	top:-7em;
	left: 40em;
  }
}

It might need a little refinement after testing on other mobiles, but it works on both my iPad and my mobile phone. The content has expanded to the full mobile screen width, and the three-line menu button is now visible top right as desired.

It works as originally intended on the desktop.

Thanks very much indeed.

How did you know that this was the cause?

John McC