SOLVED
Our expression pedal didn't work at all - a regular Line6 SP1-L6H pedal!
And then I found this video which showed a preference setting that chooses whether the input acts as a Footswitch (toggle only) or expression pedal (variable):
https://youtu.be/11EaZyiAAko?t=128
Now it works perfectly, using either mono or stereo 1/4" cable or the Y-cable, and the ControllerAssign > (Page>>) > Learn function sets it up as a controller on any parameter as expected. That pref. setting had both set to "FS" (foot switch) by defualt - fixed that and the problems are solved (after 1 week!)
