thanks, I'll take a look at these. Not too expensive too
turns out I was getting separate passes from my batch render - I was outputting a layered PSD which was showing as pure black on the file preview on my mac; when I opened it in photoshop the passes were there!
it does seem a fairly simple procedure at the start of a basic scene. I start getting confused when I've got multiple display and render layers, material overrides (which I can never seem to get rid of) on some, different lights and emitters on another .. especially if it's a long render - it's very nerve wracking - there always seems to be something to go wrong or for me to forget to check on!