iMovie will not recognise my 60fps video?
Ok I think i have worked it out. I was using ShadowPlay with my Nvidia card to record and this was the file that didn't work.

So I installed OBS, and tried the same using Software x264 to record and it works fine in iMovie, will export to 60FPS. I then tried changing it to record with hardware NVENC (which I'm guessing ShadowPlay uses) and with all the other settings the same this doesn't work, refuses to save at 60fps.

So I think its nvidia hardware recording... not 100% sure, but it seems pretty damning evidence.