I bought a DC power adaptor today, from your advice.
I got a 12V, 0.7A Max with what I believe is the correct diameter connector cos it seemed to fit just fine... but the arduino doesn't do anything. The power LED comes on, Pin 13 LED (only for a 20ms or so) which I have hooked up also comes on and the TX, RX LEDs also turn on.
The program doesn't run though. What could be wrong? I'm pretty sure the power supply is center positive, but I can switch it and test it the other way around if it is safe?
The board works fine, I tested again after with USB power.