Thanks for your reply.
This is the first time I have posted to a forum so I apologise if I don't get the protocols right.
There should be a screen capture attached to this as requested. This is a file I edited recently using the 1.9.1 version of Affinity Photo. Since updating to 1.9.2 I am no longer able to paint on an adjustment layer mask. As you can see from the History panel, I have tried unlocking the layer (even though it was not locked in the first place) and also inverting it and using white instead of black to paint with. As you can also see, the brush opacity is 100%, the flow is 100%, Hardness 0% and although it isn't shown on the screen shot, protect alpha is off, but it doesn't make any difference if it is on either. Nothing works at all when I try to paint on the mask. It seems the same problem as Studio97Visuals are experiencing.
I am probably doing something really stupid but I just don't know what it is and it's driving me mad. Any suggestions are very welcome and thanks in advance.