Why are my gas bills so high?

I live in the UK in a flat and am with N power. I don't have a gas cooker so its just the hot water and heating. I must admit its been cold here lately so have keeping the heating on quite a lot in the day, Just opened my recent bill (which i was trying to avoid opening) and its �266.12 for a quarter... That doesn't sound right to me. I have an old potterton boiler, and don't know if its to do with some setting on the boiler. Any advice would be appreciated.