So I haven't heard from support on this so any suggestions in the meantime much appreciated.

I updated my forums from 4.3.6 to 4.4.1 and it turned into a load of garbled coding so I've reverted back to original files and database from 24 hours prior.

Now all I get is: 

We're sorry, but a temporary technical error has occurred which means we cannot display this site right now. 

Invalid Template

You can try again by clicking the button below, or try again later. 

http://unofficialmills.co.uk/forums/

EDIT: I've rolled back to another day and it seems to be back.

I'd speculate that assuming the upgrader ran OK, you did run it I assume (daft question I know!) , visiting the ACP directly and running the Support tool may well of fixed this, it could of been old cached data or a concern with an existing theme you had. :)