Before boxing it up, I did some research though – the iPhone Battery life information on the Apple site explained that my experiences should have been different, I should have seen “5 hours of talk time on 3G, 5 hours of Internet use on 3G, 6 hours of Internet use on Wi-Fi, 7 hours of video playback, or 24 hours of audio playback and up to 300 hours of standby time” (from Apple site).  The mistake I had made was to assume I could charge it from the USB port on my computer, since the cable supplied with the phone is a usb to iPhone cable.  A few weeks of exclusive charge on this cable means that you are guaranteed to have poor battery performance, whereas charging from the mains on the supplied adapter leads to better performance.  I wish someone had pointed that out to me explicitly.

Empirically, I also found that reducing screen brightness was the most effective way of preserving battery life when the phone is in use, and turning off bluetooth and 3g is the best way to preserve life when you are using it infrequently.