Google Tag Assistant showed that all tags were installed correctly. A further check in the WordPress back end showed everything was in the right place, eg close to the opening <head>
tag. The Analytics snippet was re-installed to ensure it included the additional Optimize tag. Unfortunately the problem still persisted.
Heading to Google support forums, I discovered that others were also experiencing this problem. Google had acknowledged the bug and was “working on a fix”.
When coming across an error like this I am like a dog with a bone. I was determined to find a fix and assist my client.