I did have a picture frame on one of these walls and after taking
it off the back of it was covered in mildue.

My guess is a bridged cavity or failed damp proof course. If the effect
stops at a certain height, then it may be the DPC, if not, it is more likely
a bridged cavity. This could be lack of care not to drop mortar on the ties,
or could be an inappropriate insulation material (such as open cell foam)
that allows water to pass through. If the entire wall is coated, then the
insulation is more likely, as the bridged wall ties would cause a very
patchy effect.

Christian.