Thanks, Husse. A combination of spending more time on it, and your advice has got me a little bit further along.
I eventually found the proper NVidia control panel, and 60HZ was an option there, so I selected and saved it. The screen related applets within the Mint Control centre still both say 50HZ ( even after a reboot ) but the Nvidia control panel shows 60HZ, so I have to rely on that being correct ! Unfortunately, I was still getting tearing at 60HZ.
After some more research I found some advice about making sure SyncVBlank was set. Sadly making these changes also gave no improvement.
However, I did manage to find out that when I turn off all desktops effects ( i.e. compiz is not running ) the tearing goes away. This makes me think that is it something within the compiz settings that I need to adjust, but at least I have a workaround to getting watchable videos.
A task for the weekend, I think, but any other advice that others can give in this area would be appreciated