Goodness, that is pretty bad. All sorts of ways to approach this. The problem seems to be with the blue tones at the top of the image, so copy the layer and Gaussian blur (I used a value of ten).
Then use Image > Calculations with red channel on red channel with mode set to Multiply, and invert both channels. OK this and select the alpha channel it produces.
Select the paint brush and set its mode to Overlay. Paint over the sky with white, and darker lower tones with black.
Click on the RGB channel, and with the blurred layer selected, ctrl click the alpha channel you just made to load it as a selection, and add a layer mask
Wow thanks Trevor! this really is much appreciated. I've just turned my computer off but will try this first thing tomorrow.
I really am very grateful for your efforts.
Will report back tomorrow...
This really helped Trevor.Dennis thanks!
I think the fact the image is an 8bit jpeg isnt helping with this 'banding' either.
Think I need to change my workflow to 16bit.
Can I ask what your workflow is like? or anyone else like to mention theirs?
I process in capture one pro currently. Thinking I will start to process to 16bit tiffs and avoid jpegs as much as possible.
Colour and detail are important as I make huge prints...
After more research I've found this 'banding' issue is best not fixed with a blur but rather the splatter filter which you can find in the filter gallery of all versions of PS.
Lee Varis' video explains it best https://youtu.be/HLWh68q_yUI its 20 mins but worth it if you have this issue!
Add the splatter filter to a duplicate of the main image then with a layer mask brushed back the areas which didn't have banding like the shore line etc. See how much it cleaned up the top of the picture.
Hope this helps someone searching...