Switching Chrome into developer tools mode (press F12 on Windows), and choosing the device emulator, the problem becomes clear with the page scrolling beyond the right edge of the screen.
Continuing to use the Chrome dev tools I start looking for the styling culprit. A quick scan finds several
width: 800px; attributes set in the style sheets. Looking at how the page works it is clear that
max-width is more appropriate than forcing a width value. I did a find and replace in all my css template files to replace
A quick commit and republish and the page looks fine on mobile and Google has given me a tick in the box.