Chimneys can be repaired using various methods and with various different materials, in some cases chimneys may only need new lead flashing or re-pointing and new capping to the top.
We recommended applying a water proofer to brick or stone chimneys, we use Kingfisher extreme climate and have found it very successful.
Re-pointing can be done using a lime based mix or a sand/cement based mix with various different finishes.