under no circumastances do you follow the advice above and remove your own gas fire, you call in a corgi registered gas engineer to do it properly and safely.
part of having your fire serviced includes an examination of the flue (chimney) so kill 2 bird with one stone and have yours serviced if its not been done in the last 12 months.
if you want us to do it we're in the yellow pages under west heath gas, i think were in the gas section and heating section, were also in the quick find directory and on the corgi website under the find a local engineer. we only cover the south side of the city though, say from moseley, to kings heath to kings norton, across to northfield, harbourne and halesowen and anywhere in between.
A service is �40 for a fire. if your chimney did need sweeping then they usually charge around �40 in brum.