Everything seems to be logging nicely, and I've added a DS18B20 temperature sensor to grab indoor ambient temperature too. The only minor niggle so far is that the logs are not quite 60 seconds apart (perhaps the resonator clock isn't very accurate) but that's easily accounted for.

I've written a bunch of Matlab code to read in the log file and produce pretty plots like this:

Now I just need to wait for the cold weather to arrive so I can switch the heating on!