Thank you, Jerome.
That was very helpful.
On further investigation I found that Google was identifying the RUT 955 as an untrustworthy device.
Using an app-specific password resolved this and all is fine now. I'm using port 587.
Using app-specific passwords, however, does require that 2FA is turned on but by using the app-specific password the 2FA step isn't required to get the connection to Gmail working.
Thank you for your time.