Okay - looks like I might have found the problem.
It was always erroring after running for approx 30 mins.
I had a peek in to the install script:
https://rclone.org/install.sh
The script installs the regular linux-arm version for Raspberry Pi.
https://downloads.rclone.org/rclone-current-linux-arm.zip
Whereas there is a special version for RPI - linux-armv-71.
https://downloads.rclone.org/rclone-current-linux-arm-v7.zip
I manually unzipped and copied that to bin and re-ran the sync. It ran for over 2 hours and completed the uploaded error-free.
Also compared the filesize and they are different for both rclone files.
It is scheduled to run overnight again. Hopefully it should work all good again.
Thanks again for your help and for suggesting changing the version.