This is a hotly debated one. According to experts at the Energy Saving Trust, the idea it's cheaper to leave the heating on low all day is a myth. They're clear that having the heating on only when you need it is, in the long run, the best way to save energy, and therefore money. (A timer's best as your thermostat turns your heating on and off to keep your home at the temperature you set.)
The key thing to understand here is that it's all about the total amount of energy required to heat your home.
It's a given that a certain amount of energy is constantly leaking out of your home (how much will depend on how good your insulation is). The Energy Saving Trust says if you're keeping the heating on all day you're losing energy all day, so it's better to heat your home only when you need it. However, it's not quite that clear-cut. Some specialists disagree – and argue you should keep the heating on constantly for an entirely different reason.
They advocate keeping the heating on low all day, turning all radiator valves up to the max and the boiler down to the minimum, and say the problem with turning the heating on and off is that every time it's turned off, condensation collects within the walls. This condensation can help conduct heat outside the home, they say – meaning you leak heat more quickly and so will use more energy as a result.