Well after a little Googling and experimentation, I've found a few things.
First of all creating a temporary new user on my iMac shows none of these issues. Camera roll shows every time I start the photos app or plug in my iPhone.
So anyhow back to my main login.
This page gives the instructions to reset the 'Trust this computer' Reset the Lockdown folder in iTunes - Apple Support. Did all that, hit 'Trust this computer' etc and yes after plugging in my phone or starting the photos app, the cameral roll showed. But it was slightly short-lived. After unplugging my phone and re-plugging in I was back to no photos showing.
I seem to have, accidentally or whatever discovered a new workaround that at the moment seems to work every time. I have the computer set so the Photos app opens when I connect my phone, and if I connect, my camera roll doesn't show. So this is my procedure. (I had actually done the reset Lockdown folder a few times as well)
1. Close 'Photos' app (on Mac)
2. Open iPhoto - Camera roll still not showing.
3. Leave iPhoto open but disconnect and reconnect iPhone.
4. Camera roll then all shows in iPhoto. ('Photos' app opens on reconnection but still no camera roll)
5. With iPhoto open (or closed), if I go to the Photos and refresh the import tab the Camera roll all appears yay!
Still a nuisance but at least I can get it to work. TBH I believe this issue is something to with either the migration from a previous Mac OS and/or with the migration from iPhoto to Photos.