I popped the kettle on and took a look at the gas meter, to see how fast the dial spins each time I make a cup of tea. The answer is faster than expected!
One of the main reasons for this were some cold air draughts coming in through holes in the kitchen wall. How can there be holes in the kitchen wall? They were hidden behind the fridge and washing machine. The first was a large one, left in place from a previous tumble drier and half-heartedly filled with rags by a previous owner. The others were left over from the current and previous waste pipes of the kitchen sink.