Chimney sweeps and dealing with blockages

Can any one help me. I have had the makers of boiler (Vaillant) come and service my boiler and it is working fine. However, the hot water keeps switching off and the guy said that it is due to a block in the Chimney and the gas(smoke emission ) is not getting out hence the senser turns the boiler off. Does anyone know who should fix it ? Is it the guys in Black that come to check it every so often ? and if it is these guys does anyone have the telephone number for them ?

Any advice v.much appreciated

(Boiler is my responsiblity - I own the flat)

This might help (didn't read it through but they talk about chimneys and boilers): Sweeping the Chimney

Try this for a list of chimney sweeps in your area. Kaminkehrer

Thanks that great - have called them and they come friday