Thanks for the comments. Looking on my laptop the corners are way too dark.
Whether it is;
moving from a wide gamut monitor to a laptop or,
working in Gray Gamma 2.2 and then converting to sRGB or,
working with a 16MB file and squeezing detail out of shadow areas and then expecting the detail to be as visible in a 1050 px high image or,
it was too dark in the first place.
I will spend many frustrating hours trying to match what I know prints well to what looks good on a variety of monitors, my monitors are profiled. I may look at working out how to get my wide gamut monitor to swiitch to sRGB that should help.
Any help gratefully appreciated Ken