damp internal wall

Hi
My master bedroom (upstairs) has an exposed external wall which attracts damp on the inside. The wall is pebble dashed and is of solid brick construction. The plaster on the inside of the wall has become ‘live’ in a number of locations where it has detached from the brick wall. Mould has also begun to appear. I fitted a ventilation duct to the outside the other year but the problem still occurs. Not sure if the prob is condensation or moisture penetrating the brickwork.
Where do I go from here as far as treatment to this wall?
As usual, any advice gratefully received.

Is the pebble dash in good condition, are there any cracks, is it blown? Is there a chimney on the wall? Is the roof in good condition over the top of the wall? at what height are the damp spots? It could be condensation or it could be ingress of water from somewhere. More investigation is needed.

thx 1 John

I don't believe there's any damp getting in now. It's a solid brick wall and exposed and I'm sure there will be moisture somewhere so I've screeded the internal wall with a sand/cement scratch coat with waterproofer. To complete the job and to apply the finishing coat plaster is it correct to now bring up the level with bonding or to use more sand/cement??

Thanks in advance all !