If you use your open fire often over the cooler months, you should have it cleaned every year. In theory you could do it yourself, but you risk damaging the lining, particularly in an older house.
Contact The National Association of Chimney Sweeps (www.nacs.org.uk) for recommendations. Once it’s been swept, you might want to try a maintenance product such as a Chimney Cleaning Log. Although it doesn’t cancel out the need to sweep, it softens and removes tar deposits and reduces the risk of a chimney fire.
Another tip for reducing tar build-up: burn well-seasoned wood and/or smokeless fuel.