Chimney are swept based on the assumption that the flue and chimney terminal (pot or birdguard) are in a good enough condition that they can be swept. Pro Chimney sweep can not be held responsible for any damage caused by poorly fitted pots or bird gaurds or any other parts that are the responsibility of the stove or fireplace builder or installer.
Older Chimneys are prone to loose morter and this can still fall after a chimney has been swept , this is quite normal.
There are specific guidlines regarding sweeping chimneys in nesting season please visit the link provided to make sure that your chimney is safe to sweep