Updated
Wood burners: at least twice a year
Wood gives off more tar and moisture than smokeless fuel. As the smoke cools in the flue, that condenses on the walls as soot and creosote — a sticky, flammable deposit that narrows the flue and is what catches light in a chimney fire.
HETAS encourage having the chimney swept at least twice a year when burning wood. If you burn a lot through the winter, or the wood is not properly dry, the flue builds up faster than that.
Smokeless fuel: at least once a year
Smokeless fuels leave less deposit, so HETAS put the minimum at once a year. Once a year is a minimum, not a target — a flue that is used every day all winter is a different case from one that is lit at Christmas.
When to book
The best time is just before the heating season starts, so the flue is clear before you need it. If you sweep twice a year, make the second one after the peak of winter.
Also book one before you light a fire or stove that has sat unused for a long time. Birds nest in chimneys that have not been used, and debris falls down a flue that has stood cold.
Signs it needs doing sooner
Smoke coming back into the room, a fire that is hard to get going or will not draw, a strong smell from the fireplace when it is not lit, or soot and debris falling into the grate. Any of those is worth a sweep before the next fire rather than after it.
Keep the certificate
Ask for a sweeping certificate every time. It is your record that the flue was swept, and it is the first thing an insurer is likely to ask about after a chimney fire. We leave one after every sweep.