Battery saga update: I bought a second battery at London's Apple Store on Regent Street over the weekend for £97 (about $145). My first battery has recovered its ability to retain a charge and now clocks just over three hours regularly, with brightness dialed down three segments from max and BT off/Wi-Fi on, using it mainly for browing, e-mail, Skype and news reading.
Interestingly, the new battery, which has not been calibrated of course, shows the same time left as my first battery, about 2:12-2:20 minutes after a full charge using identical params as above.
So I guess that in hindsight Apple was right to refuse to replace my battery. It seems that the MB battery has memory problems, so my frequent partial charges left it incapable of discharging at its peak rate. I now let it run down to 0 minutes all the time and that has helped a lot.
It also throws into question all those reports of "five-hour" batteries, which seems so far removed from reality, based on my experience with two batteries now.