Hey Mario, thankyou so much!
I've just now followed your instructions very carefully, and hey hey what d' yer know!
I also had a play with other knobs than vol just to see what might happen, in order to see how else I might be using this routine down the track, and I think the light in this thick head is slowly coming on.
Thanks especially for the "baby steps" recipe, because it helped me both achieve my immediate purpose and led me to see where my previous idea about the MIDI-CTL association routine was completely wrong.
Clinton.