In regards to my original question, I have been majorly distracted with other work. Back at in now. I'm not sure if I am on the right thread though and its a bit redundant now, as after many, many instances of it not working (re-boot, re-load not helping), suddenly it is fine, but other little bugs have popped up. Specifically, I would load a colour photo, convert it to black and white, then re-convert to RGB. Then on the adjustment panel I select Gradient Map and then the pre-set that I put there ages ago (which worked just fine for months). When I first posted this, at the time, I was lucky to get it to respond to any of the defaults, never mind my preset. I am (and was) working on a MacBook Pro OS Big Sur 11.6. My best guess is that I uploaded the latest version of Photo a while back and that has resolved the problem (but I'm still nervous).