Update: I have now solved the problem, which I believe is related to the Apple Devices App not being configured to allow .MOV files to be transferred from PC to iPhone (even though they are iPhone Created, not manipulated, MOV Files).
I managed to fix the problem by completely removing the Apple Devices App, Music App and TV App from my PC. I then carried out a full reboot of my PC and Phone. Downloaded the latest version of iTunes and once I had plugged in my phone, clicked on the phone icon at the top, to my surprise, the whole old options became available again (sync, backup, music etc.) - which I didnt think was possible, as I could not find it - when Apple Devices was still loaded.
I then Deleted the "iPod Photo Cashe" from my Photos Drive on my PC. When I selected to sync my selected photos again. After a full backup it loaded all the Video Files back to my iPhone. The same as it had been doing before I switched to Apple Devices (instead of iTunes).
Thanks to everyone who tried to help.
A Lesson learnt, stay with what you know works...lol