A chimney should be cleaned at least once if not twice a year dependent upon the amount of usage it gets.
You can find a chimney sweep from the following websites: -
National Association of Chimney Sweeps – www.nacs.org.uk
Sweep Safe – www.sweepsafe.com
The Guild of Master Chimney Sweeps – www.guildofmasterchimneysweeps.co.uk
Association of Professional Independent Chimney Sweeps – www.apics.org.uk
ICS Chimney Sweeping Association – www.icsweeps.com