As mentioned, my S60 doesn't detect any wi-fi signals except my phone's hotspot – no networks appear in the list. I suspect that recent Volvos have been changed so that they don't detect weaker wi-fi signals or signals from outside the car. I found this in the Owner's Manual:
"The car’s Wi-Fi system is designed to handle Wi-Fi devices inside the car." (p.515)
I've tested the S60's wi-fi hotspot. Just like a mobile phone, it turns off wi-fi ! So, the only way to connect to the internet is by a Bluetooth connection to another device. Why bother having a wi-fi hotspot in Sensus if the car can only connect to a Bluetooth device i.e. a mobile phone ? Surely, if there is a mobile phone in the car, owners would connect to that instead of connect to the car hotspot which in turn is connected to the phone ?
The local Volvo dealer service manager thinks the S60 should connect to any wi-fi network. They use a wi-fi network connection when servicing the car. I'll test that next week.
So, this is still a work in progress. I might get a wi-fi range extender and blast a signal at the S60 from inside the car (my home wi-fi signal is 5/5 strength already). For now, I'll do smaller updates via the mobile phone and navigation updates by USB stick.
Many thanks all.