Funnily enough, Fring was one of the apps I uninstalled that was running needlessly, so that's obviously helped too.
The reason I think it was tweetcaster though is I'd had fring installed for a while and I only started using tweetcaster the day that I had the problem. It's the only thing I'd changed that day and the drain on power was pretty dramatic.
That's one of the reasons I wanted a battery app that gives specific power consumption for each app. I installed the spare parts app listed above, which is good, but only lists the time the app has spent running rather than how much power it has used in that time.