The most common cause of blocked chimneys is the construction of a bird nest in the flue. These are removed by sweeping them out from the bottom, not from the top of the flue.

Typically the nest is constructed at the level of the top floor ceiling height,about 3 metres from the pot.

If not removed, nests can be damaging to the fabric of the chimney and prevention of nests should be carried out after removal.

The cost of removal varies depending on circumstances.  Removal of nests can only be carried out after fledglings have departed.

I can advice as to when removal is appropriate