It’s ground floor, chimney wall left side and outside wall.
Theres a gap in the left corner when I can see the chimney brick foundation.
Thinking of sealing this gap.
Thanks
Fireplaces generally have a hearthstone which is laid on the ground or rubble fill. Neither the stone nor the base of the chimneybreast are likely to have a DPC, thus damp creeps in. When the fireplace was in use, that did not matter, because frequent fires would help dry out the moisture, and airflow up the chimney would carry away the water vapour. Neither of those factors applies any more. Furthermore, disused flues that are unventilated are prone to condensation from the warm, moist air in the house. Adding sealant will not remove moisture. Ventilation can.
An opening at the top, plus an airbrick into the room at the base, will often be enough. But bricked-up fireplaces often have a mass of damp rubble hidden inside where the builders couldn't be bothered to barrow it out. As it is in contact with the damp hearthstone and brickwork, you need to remove it. You will see when you make your initial ventilation opening. You may also find damp rubble that builders hid under the floor.
For best results, open up the fireplace enough to break up the hearthstone and dig out the rubble, below floor level. You can then make an opening in the base of the chimneybreast enabling it to ventilate from the subfloor void. This will have the added advantage of drying out moisture in the void, and you will not need an airbrick and draught in the room.
This is not difficult or complicated, but the time and effort is more suitable to a DIYer who has the long-term benefit to his home in mind.
You mention it's an outside wall, so check outside for water coming from a leaking gutter or downpipe, earth or paving piled up against the brickwork, broken drain, or other source.
