Well I managed to fix the problem

In order to minimise the number of original rome files that I'm overwriting for the mod, I had changed the texture file references in the wonder and symbol_ CAS files in the models_strat folder. To do that, I'd simply opened up the CAS files in notepad and changed the reference towards the end of the file. Apparently the game doesn't like that. Oh well, everything's fine again, so I'm happy.