About ten years ago I had some plastering work done with sand /
cement / additive plaster about 1m high across the front of the room,
and the rest of the room just skimmed over. There is a persistant damp
area above the line of the new plaster going up to ceiling height and
appearing to a lesser extent in the hallway by the front door on the
other side of the wall. Paint has 'bubbled' and some of the plaster
skim has blown away from the wall.

One of the walls affected is cavity which has been filled with loose
insulation, but the other can only be half a brick thick as it is the
internal hall / front room wall extending past the front door where
there is an open porch about 1m deep.

I am fairly sure that the damp is condensation but no other part of
the room is affected. There is a radiator under a bay window to the
right of this corner but being in a bay might leave the affected walls
cold. I have installed trickle vents over the double-glazed wooden
sash windows and the room doesn't 'feel' damp.

If necessary I can get at the cavity wall by removing the timber which
closes the cavity where it turns into the bay, and drag out the
insulation and check for water ingress, although obviously I'd rather
not! Shoul I re-plaster, or is there something I can treat the plaster
with? Should I insulate the half-brick wall outside the front door,
and if so how?

Any advice gratefully received

Peter