Tom,
I really don’t think any other plugins are casuing the issue. Last week I disabled them all and then turned them on – one by one. The plugin that caused the issue was the GP Premium addons.
Now if I switch themes, will I lost all my GP customisations? I’ve put a lot of work into them and I would not want to lose them. Also I am using a child theme so that part of the customisation is all good but my major concern is the ‘Customiser’ tool.