If the wall in question, is the one facing the park (tree'd area, and its toward the front of the house) then its possible that damp is penetrating thro the corner, as moistire will be sucked up the large 'garden wall' at the front by capillary action, and as it is abutting the dwelling up to a height of maybe 6ft, that will transmit damp through, well above any DPM in the house. End of terraced houses are the worst, the gables cause trouble, folks throw stuff over the wall, and its facing the wetest windiest side. Also you will have 3 exposed wall, instead of 2so house will be a lot colder, and west is the worst way to face a wall.