I actually had the Gradient Mapping already, but it didn't seem to do what I wanted it to do and it was veritcal only (or so it appeared). I'm probably just not using it correctly. I tried Gradents Galore and it was pretty close to what I was after.
Thanks BoltBait! This was a lot closer to what I was after.
I must confess, in the interim I found an existing rainbow gradient with a texture applied and used it instead (my particular task was to colorize a white wave form of a sound sampe with a rainbow hue/effect). But I've got all the new plug-ins to play with now.