Ok I've been at this for hours now:
Firstly I deleted all the names on the photos of the person who went missing during my attempts yesterday. Then I started again, the option to "make key photo" now appears in the contextual menu - hurrah. Must have been some weird glitch.
I am running the latest versions of ioS 11.4.1 and High Sierra 10.13.6 . However I can not sync my entire library from the Mac on iCloud since it is too large to do so. So I am syncing via iTunes.
Here are the other issues I am having:-
1. Syncing doesn't always sync new Albums created on the Mac - I can't figure out how to force this to happen. I know I did make it do so once but think it might have been by deleting and starting again!
2. Folders within Albums on Mac wont appear on the iPad
3. Smart Albums do not sync to the iPad
4. Albums that have synced onto the ipad, appear in a totally random order.
5. "People" on iPad does not seem to relate to People on the Mac
6. "People from my Mac" on the iPad seems to have old people from when it used to sync with iPhoto...and it seems there is no way to delete these.
7. "People from my Mac" - on the iPad, chooses a random key photo which I then can't change on the iPad.
8. I can neither merge nor delete "People" on the iPad.
You can see why I am so frustrated.
Thanks for all your help
curlydog