Have you tried the M key? It switches between showing the mask and showing the effect of the mask.
Aha! How stupid do I feel? In fairness I tried M, but when I looked at the on-screen help it says shift+M. Also, I had assumed that this would work the same for all types of adjustments (i.e. it reverts to removing the mask when you move a slider), but it seems the behaviour is different for the Control Points.
Anyway, thanks very much Jim.
So as the problem is solved, I’m closing the issue if you do not mind