I can't seem to figure out the set-up required, ie the combination of renderer and file type/format, to allow the production of a masking channel when rendering a scene with various objects, or even one! I want to be able to take a rendered scene or grouped obejct into a Photoshop file and isolate it for combination imaging but can't seem to work it out. Any ideas?

I have yet to do this but it looks simple enough. In your render options you shold see something for G-Buffers. If you choose .tiff for pc or .pict for mac (or photoshop I assume) you should be able to select an alpha matte preservation option. I don't understand what an alpha matte has to do with G-Buffers, but whatever :)

Dear Brenthomer, many thanks for your reply. I'll give that a go. All the best.