In short, it seemed that Symbian didn't like the so-called 'PSK-mixed' mode beloved of many home wi-fi routers. This is also shown as 'WPA-PSK(TKIP)+WPA2-PSK(AES)' mode, as shown above.
Essentially, this is a mode which is backwards compatible with devices that are quite old and only have 'WPA' compatibility. With modern smartphones all fully capable of handling WPA2 encryption, I suspected that this compatibility setting wasn't strictly needed. Yet, somehow, I'd not tried the 'AES only' option, shown checked above.
Having changed this setting so that my router was running in pure AES-encrypted mode, I found my Nokia E7, which had previously been almost unuseable on Wi-fi, suddenly became rock solid at downloading large files (e.g. podcasts) and I wonder whether this router change will also help others with N8s and C7s which have been somewhat glitchy on Wi-fi?
To check your own router's settings:
- Sit down at a hard wired computer, i.e. one connected using Ethernet to your router.
- Log into its admin control panel by going to '192.168.0.1' (or similar, see your manual/help pages)
- See the manual/help for the appropriate admin username and password, if you don't already know it for your router
- Look under 'Wireless settings' and switch it to a security mode of pure 'AES' encryption.
- Click on 'Apply'.
You may find that you also have to turn Wi-fi off and on again on any devices or phones, so that they pick up the new security mode.
Update: after being rock solid for two hours, I restarted the E7 'just in case' and all of a sudden it's as flaky as it ever was on Wi-fi. There's definitely something conflicting between WPA2 encryption and certain Symbian device Wi-fi stacks.... Sigh. Something tells me we'll never get to the bottom of this....