This is a hotly debated question, even amongst plumbers and heating engineers. Which is the most efficient way to heat your home, keeping the heating constantly on low or only switching it on when cold? There are pros and cons to both options.
No home is 100% insulated; there will always be a small amount of heat leaking out of your home. Keeping your central heating on all the time means there will always be a small amount of energy escaping from your home. This will be felt more in a poorly insulated home, as the boiler will need to expend more energy to maintain the indoor heat.
However, if you only switch the heating on when you need it, for just a few hours a day, the boiler has to work harder to heat the house from cold than it does to maintain a constant heat. Turning the heating off, letting the air cool down, causes the air to deposit water droplets in the form of condensation. If this collects within the walls, it causes heat to leak more quickly losing more energy. There are ways around this, but the most common way to combat condensation issues sadly is to keep your heating on and ventilate (usually your windows left open).
In most cases, it will be cheaper to only put the heating on when you need it on; there is no sense in heating an empty house. Although, it really does vary from house to house which is the most energy efficient.
Aside from the energy aspects of the debate, keeping the heating on all the time instead of waiting until it is needed, means you will never be cold. If your house is well insulated it may actually be better to keep the heating on low constantly.