I gave up on solving this - I don't have the knowledge/skills myself & nobody on this, or other forums, seemed to have any solutions either. In the end I uninstalled Office 2016 & installed Office 2019. The company I bought from then refunded me the cost of office 2016.

2019 seems to be working fine, so I'm satisfied with that, though a bit frustrated at not being able to sort it - I have another W10 laptop that has office 2013 working on it just fine, so the company's assertion that it was because 2016 is too old just doesn't hold water. I guess this'll just have to be one of those IT mysteries...