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 - my next step once i get the simple diffuse working consistently.
Also - I just noticed (by peeking inside the DAE file, that the OpenCollada export plugin for Max2016 is version 1.6.xxx and i read somewhere that SL only likes 1.4.xxx. The internal MAX exporter (via FBX export), creates a 1.4.1 collada so I should probably stick with that one.
@arton Rotaru Thanks for the tip - i don't understand why it's needed but I'll keep it in mind. The FBX exporter already has a re-triangulate option on export - is your tip just a precautionary measure or is there some other reason to double up on triangulation?