I have no idea what's going on now. I retraced my steps and did exactly the same thing in a new scene and this time the manually applied texture worked fine. I suspect there's some "strange" intermittent issue with the beta grid. Of course I'm still having issues with the textures uploading WITH the model, but I can live with that as long as the UVs don't get messed up during the upload. Last night I had normal map issues, where the normal map was being applied as a standard texture on the face of the cube and not distributed as per the UVs on the model - dunno what that was about either -