Ferroli HE31C - Hot water but no heating?

There is an error code D2 on the boiler and no matter what I try I can't make it go away. I've tried turning it off then back on again and it seems to start firing up but then goes straight back to D2 again. Everytime I change my thermostat to 20 degrees c it changes itself back to 15 degrees, and i'm freezing, and I have a cold! It started when the pressure dropped, managed to get the pressure back up between 1 & 2 but since then i've had no heating.