OK I've been doing some more testing.
I tried escalades extended build but there was no change to my refresh rate issue. I also tried a couple of different Milhouse builds but these are also not working for me.
Next I tried LibreELEC 7.95.2 and 7.95.3 but both of these have the same issue as 8.0. Then I tried LibreELEC 7.0.3 beta and this works fine - as does OpenELEC 7.0.
I have read a few threads where others are having HDMI related issues so I thought I would try connecting the NUC directly to my TV rather than looping the HDMI through my Yamaha receiver. This works perfectly with all the LE 8.0 and also escalades build. So the issue seems to be every build of LibreELEC with Kodi 17.
I have created a custom xorg.conf file as detailed in the wiki page and this means I now have a couple more refresh rates available but it does not have all the modes that I need so this is only a temporary fix as far as I'm concerned.
I really like Kodi 17 and the new Estuary skin so I don't want to go back to the older versions.
Is this issue likely to be resolved any time soon?
Please can someone give me some info??