Verified quite a few times over the course of trying to fix it, I don't appear to have had any issues that others haven't reported already.
I.E - Object Reference Not Set to an Instance of an Object - and - Null Reference Exception (these do sound like they could relate to missing files!)
I think the PC that has missing files may have been downloading from another PC in the house rather than getting fresh copy, apparently Steam does that now.
I'm going to uninstall from that PC and try again as its just an old one I used as a dedicated server many alphas ago, now just used as Netflix PC on my bedroom TV.
Thanks for the response, I'll let you know if a (real) fresh download restores the missing files.