Thanks stokerg. However, I realized my mistake last night. I had got in a muddle over the layers, and the wrong layer had been selected before the run. When I corrected this I got a proper looking spectrum, and reducing my brush size to 45 for the dark mask spots, got a really excellent result. As the FFT Denoise feature was the prime reason I discovered Affinity Photo and subsequently downloaded a trial version, I was very happy! - and I will now buy it. I had looked at other ways of implementing FFT on my textured photo scans, but I got wary when I saw how much fiddling with Windows parameters via command lines was involved - too risky, I thought. I think that the tutorial on FFT Denoise could be beneficially expanded, for example, what kind of layer should be preselected.
I also think that this powerful feature should be given more prominence in Affinity Photo's promotional material, because I believe that neither PhotoShop nor PaintShop has it embedded or available as a convincing sounding plug-in.
