Anyway, I hooked the immersion heater up to my arduino thermostat, with the vat snug in its wet suit, and off it went. I did, however, however, have a slight hiccough with the thermostat. Before plugging it in and leaving it, I decided to do a double-check that everything was working fine. It wasn't. Long story short, my power supply wasn't stable enough to form a stable reference voltage, so the temperature reading varied by +/- ~15 degrees C. Ridiculous... Anyway, I plugged the arduino into my laptop, plugged the charger in, and left the whole lot looking after the beer.
6 days later, it was bottling time.