I'm doing my best to sort it out. So what happened?
My hosts insisted I upgrade from php 5.6 to 7 to save load on the server. This is good for the main site but not good for the forum.
Turns out phpBB 3.1 which we were running isn't compatible with php 7, hence the brokenness of everything around here.
phpBB have released version 3.2 which is compatible with php 7 BUT it's not backwards compatible with 3.1 MEANING that the theme, extensions and modifications don't work when I upgrade (which I've tried for hours today!).
What am I doing next?
Well the shit show is only going to get fixed by:
Clean install of 3.2
Updating mySQL database so its compatible
Installing the old theme (or new similar one if its not 3.2 compatible)
I'll port all the images over and the database is safe.
It's a cunt but it's our cunt. As they say. Nowhere.
Editor, United Rant