Thanks for the suggestion Ariba.
I added the suggested CSS within the Custom CSS section of Styles > (Applied Helix Style) Custom Code > Custom CSS to no avail.
For what it's worth.
Zooming in at 110% the menu looks fine but more than that, it also overlaps like when the browser is reduced in size, then finally collapses to the off canvas menu when zoomed in at 200%.
I have changed my main menu item to be a very long article, so I know it’s no other content / items on Page Builder effecting the display of the menu.
Thanks