Thanks Edward,
It's the second method I had been trying to use, but I couldn't see how to actually select individual sub-masters - a bit of experimentation has now revealed that I should've been using [SHIFT+FLASH BUTTON] to make the selection.
E2A
So the behavior I get now is (1) run a number of chases (2) select sub whose speed I want to change (using [SHIFT+FLASH BUTTON] to make the selection) (3) hold [SHIFT] and adjust the speed control and the speed of the selected chase only changes but only until I release the shift key, then all of the running chases change to the new speed.