I have a house (victorian terrace) that has several wet patches on the
walls downstairs (kitchen and through dining room)

These patches come and go, but are always in the same place.

1) In the kitchen around the edge of the tiled splashback, and under the
work surface.
2) On the chimney breast
3) On the wall next to the chimney breast (not wet but can seen a change in
colour on the walls that comes and goes)

The patches appear 2 metres or higher on the walls. I have previously
removed the plaster and replastered the area on the chimney breast (2), and
fitted an airbrick to the breast.

The wall next to the chimney breast backs onto the shared passage, therefore
the wall should be quite dry, and unlikey to be penetrating damp. I also
understand that it cannot be rising damp, and there is no dampness at floor
level, but higher than 1m.

The wall in the kitchen is similar not an outside wall, and I can see no
reason for moisture penetration.

The house is fully double glazed, so the walls may be the cold spots. Could
this be caused by condensation? particulary as it directly adjoins the
kitchen. What would be the recommended methods to resolve this? Would a
dehumidifier be useful in these circumstances. Any other suggestions?