Chimney breast dampness

BarraOG

My parents have evidence of dampness on the chimney breast of their house (see attachment). There are bubbles on the wall in the sitting room above the fireplace. These bubbles are located randomly on the wall and can be seen from where the wall meets the ceiling all the way down to the mantelpiece. Why is there a ledge like that on the chimney? Would a local builder be the right person to get in order to solve the dampness issue? I presume this will involve sealing off external ledge. Will it be a big job in your opinion?

Calahonda52

Picture not much use.
You don't give any clue as to where the dampness is.

One place to check is for a crack in the chimney cap around the flue, allowing water to run done the outside of the flue down the stack and then come out where the flue changes direction about 5 to 6 feet off floor level inside

BarraOG

There are bubbles on the wall in the sitting room above the fireplace. These bubbles are located randomly on the wall and can be seen from where the wall meets the ceiling all the way down to the mantelpiece. Didn't see any cracks but I'll need to get up with a ladder to be sure.