Update 14 August 2012: The wifi problem is still ongoing, with the wifi connection being lost erratically – sometimes when haven’t used browser/net for a while and sometimes when still using it (which is even more annoying!)
I’ve tried all the different suggestions so far, so looks like it will be a similar situation to last time – the wifi connection problem wasn’t fixed until Apple released an update, that time to Lion, and it was specifically ‘iMac WiFi Update’. Come on Apple, sort it out!

The list of other users with the same problem is mounting too – see Apple Support/Forum https://discussions.apple.com/message/19135664#19135664

Update: After 90 minutes of use, the wifi connection still hanging in. I’ve been using browser, then using a totally different non-internet application, then back to browser.
I did have an initial problem where it dropped out straight away, but this was fixed when I  pressed ‘Turn Wi-Fi Off’ and then On again in Network Preferences, to ensure I was using the new wifi settings – with MTU change.

Original: After previous attempts (WEP to WPA2, DHCP clear) at fixing the wifi issue experienced since Mountain Lion upgrade, I’ll now try another fix – Changing the MTU size to 1453.

I’ve now made the change to my system, so will report back after using the system for a while.