Thanks for that.
As I say, I seem to have it working ok now, just me being dim, trying to run before I could walk. Once I'd read the chapeter in full had it cracked, really quite straightforward.
Fine with JPEGs, which is just as well as we never used TIFFs, only EPS previously, but virtually all abandoned now for JPEGS.
I'm turning my attention to colour channels next; watch this space