I emailed Polar support who gave me this advice, which worked:
1. Uninstall Bonjour (Apple software, which is installed when Flow is installed).
2. Uninstall Polar Flow
3. Reboot computer
4. Install Polar Flow
5. With Flow running, connect V650, switch V650 on, wait a few seconds
At stage 5, my V650 was detected again and everything updated automatically. I was ready to sell my Polar V650 and get a Garmin. This obviously has taken a lot of hassle out of the process. Thanks to my wife for telling me to just email Polar directly.