Hi I used City Sweeps
www.citysweeps.london
or ring 020 8241 3213 or 07972 105045
The guy there is called Ed and he is really helpful, punctual and clean and also services Wood Burning Stoves as well as being fully insured and a member of the Guild of Master Chimney Sweeps.
Watch out for people who say they are chimney sweeps but are not qualified, apparently its not a regularised industry, National association of chimney sweeps and Guild of master chimney sweeps are the only organisations i think that are recommended by the government body HETAS.