UV Mapping - FBX meshes wont accept textures

I've imported (from Autodesk Inventor & Cloudcompare) a series of meshes, set them up in my scene, got all the positioning sorted, colliders etc. All great, but I cant apply textures. From searching around I think it must be due to the FBX UV mapping, as I can dump a texture on a Unity created cube no problem.

Neither of my modelling sources have the ability to export the FBX with any options so I'm stuck in Unity.

Can I fix this in Unity, maybe with a script? I foolishly thought I'd leave the 'easy' bit of texturing everything until last.....

I'm really hoping I haven't wasted days building my Unity scene if I've got to find some other route in for all the meshes..... there's hundreds of them :(

Thanks,