Just done that and no change. After I've cancelled through all the error messages, the game plays normally.
Ah, might have just found the cause. I've loaded the model into Fragmotion, and it says:
Error loading texture: D:\DarkMATTER V2\Geo-Metricks\Model Pack\Actors\Monsters\Assault Trooper\distgrtr.bmp
Error loading texture: D:\DarkMATTER V2\Geo-Metricks\Model Pack\Actors\Monsters\Assault Trooper\cywarr.bmp
So it looks like some texture information is stored in the .x file and the system obviously can't find those files. But that directory structure and those files don't exist on my home PC either. Strange how it works correctly in the DM2 browser though.
I've re exported the model from Fragmotion and the errors have disappeared, so I'll just need to find the other affected models and it looks like it'll be back to normal