Thanks. That got me part of the way. The Acer will now see and recognise the network, but it still won't connect. The diagnostics say "Problem found. problem with wireless adaptor or access point.", without saying how that might be fixed. The Adaptor is a Broadcom 802.11n. Its properties say "This device is working properly", so I assume that means the problem is with the access point. There's plenty of signal strength but IPv4 and IPv6Connectivity both report "Not connected" and there's almost no sign of traffic on the router.
The router/modem is connected to the Internet - otherwise I couldn't send this from my ethernet connected desktop PC.
Could this be soemthing to do with the change from WPA in the old network to WPA2 in the new one?