Is it a pitched roof, a flat roof, how high is it, what size is the chimney stack, will scaffolding be needed to get at it, does the crack go all the way round the stack horizontally or is it a vertical crack? Is the stack in brickwork or blockwork; is it rendered or can you see how it is constructed?
In short, you really need to provide more information and, I would suggest, you need to be talking to a builder or three to get some idea of how much it would cost to repair, if indeed it needs to be repaired. I would, if I were you be asking for recommendations for builders in your neck of the woods.
Good luck.
BillK