OK so I allowed it to switch to Replace/Update and it ran through. It's delivered an improved result to the RGB combination, but the bottom-left triangle where the widefield image doesn't cover is still awful.Hi John and thanks for the suggestion.
I found an old DSLR image of M31. It's not a full overlap:
View attachment 12153
so when I got to step 3 above, PMM gave me the message:
View attachment 12152
Noting your response to Wei-Hao, I did not take this option, so what should I do? I've tried ImageSolver as a pre-requisite to MosaicByCoordinates but it can't solve the DSLR image. I assume the registration process has upscaled its pixel resolution to match my main image so I entered the same resolution in ImageSolver.
I guess the best thing will be to take a fresh widefield image. I think that's the only way I'm going to get a linear one.