To cut a very long story short, we tried out Google Optimise. Since then the bounce rate (even on the baseline page) jumped from 3% to 85%.
Discontinued Google Optmise, but the bounce rate still remained high.
Read that it can be causes by duplicate analytics code, but I have done a search of the home page on developer and only one code on site.
Done cross browser checking. All fine.
Any ideas of where to go next? Bounce rate has been hovering around the 85% mark for nearly two weeks now.