Hello Virk, thank you for your answers and suggestions.
I did disable ALL plugins - but the problems persist before reactivating even one plugin. I thought perhaps I should re-publish a page or two in order to override any cached versions, but even after republishing the issues are still there, notably a warning: "A PCRE internal error occured. This might be caused by a faulty plugin" .... which is hard to understand when all plugins are disabled!
So I'm inclined to think it's a more fundamental PHP version issue.
For next step I will follow your suggestion of installing a fresh wiki (Jackrum) and copying the data folder over. Then try and reinstall plugins, some of which have been updated since I originally installed them.
Ultimately my fault for leaving something to lie fallow for so long! But I'm just looking for the quickest, most efficient way to update to modern server requirements.