Thanks for all of your help,
However I’ve decided to just create a new folder and do a clean full install of Joomla 3.8.0 and reinstall everything. I have configured the template and duplicated it for all of my languages. In addition I’m about half way through installing my extensions & components & the problem has not appeared.
I will keep you updated on my progress, so if the problem does appear hopefully we’ll have a good idea what’s causing it. Right now my guess is that something went wrong with the upgrade.