If Papers, Please has taught me anything, then it's that you should get food and heating on alternative days to save money. But I don't think that works out in real life so well.
9 hours a day of central heating though is utterly crazy. Either your house is incredibly poorly insulated or your housemate is wasting energy somehow. Get her to invest in some better insulation, you can get window insulation really really cheap and you just stick it on the glass. And get your housemate to invest in a pair of slippers or something. Or just get her to pay more of the bill, maybe.