UPDATE
The day I made post above on Tuesday, I switched off and unplugged my Roku Express, PC and router for around 20 minutes, and when I restarted, everything was working fine again - joy!
It was working well for the next few days, but then the same thing happened yesterday (Friday), only this time, shutting down everything and restarting (numerous times!) has had no effect so far. Symptoms are the same, i.e. my Roku Express simply can't "see" my wi-fi network, though it can see all my neighbours' networks!
However, as part of my troubleshooting, I gave my old Roku LT box a try, and it worked perfectly straight away, no trouble at all with the wi-fi. This indicates to me that it's the Roku Express itself that's the problem, and not the wi-fi... I emphasise that everything had been working perfectly for months (forever!) before Tuesday - are Roku installing firmware updates which could be causing this?
Thanks again for any help!!