The cost of our house's heating

Our house's gas supplies the oven and heats the water but, most of all, it fuels the central heating. Last month's energy bill has us consuming quite a lot, two hundred megajoules per day on average, and yesterday it stayed below freezing outdoors so I expect this month's bill to be no better. I further suspect that the house's heating system needs replacing but I am content that our landlords have not raised our rent since we moved in. I now find through experimentation that although I have the thermostat set at 64°F the house stays above 68°F. Now I have the thermostat set at 60°F the downstairs is actually managing to nudge below 68°F so at least there does seem to be a connection between the thermostat setting and the resulting temperature. I hope that we now start to spend a little less on energy.