Problem seem to be with a rogue ad (Morrisons), which broke the layout and hid the navigation bar (and page footer) completely: I can still see the bar content in the page sources.
Problem will therefore (most likely) affect only UK users ...
Changing the User Agent string from Default to something else does nothing, but when changed back to Default it will refresh the page and display correctly.
On next refresh the problem is apparent again...
Error console shows nothing. Sometimes on refresh the ad will be displayed at the bottom of the page (as opposed to the top of navigation bar) but still
"swallow" both navigation bar and the page footer.
Bit annoying 🙁
Trebor.