does anyone have any idea what, missing from a BI modfolder would cause an errorless CTD when you press "start battle" on a campaign map battle?

it seems to work fine for custom battles, and loads to the extent of seeing and deploying the army, whichever side you start as, just CTD's on pressing start.

I know it's because I've been trying to thin out the BI modfolder to try and avoid copying / sending everything, just have no idea where to start looking for what might be doing this... only tested intermediate stage on custom battles and everything seemed fine....