Update:
Guys, following on from my posts above, I'm now using iOS 13.2 and Mac OS El Capitan 10.11.6.
When I upgraded to iOS 13 I made sure I didn't upgrade the Reminders app as I knew if I did I'd be unable to sync reminders between my El Capitan and iPhone.
All was working as it should up until about a week ago. But all of a sudden, if I amend Reminder dates and times on my Mac that amendment doesn't cut across to the iPhone. However, if I amend Reminders on my iPhone they do cut across to my Mac.
Any ideas what has gone wrong please?