Thanks for taking the time to look. I’ve actually gone in a different direction with a plugin. But I did manage to identify the problem in case you’re interested.
Having painstakingly tried with positioning etc and sweeping across the screen with the cursor in inspector for hours, I found the svg icon for menu was invisible/not showing, but clicking on it brought up the menu. (It did not work like this in the live mobile version, just wordpress mobile preview in Customiser.) I tried finding the svg icon for the menu identified in Page Source but just could not get a solution (used svg allow plugins, z-index didn’t help either). This occurred both on Safari and Chrome.
Went with a plugin for mobile menu and now flying with the theme!