100 year old house. Wall 12 ft X 12 ft newly plastered. I had originally taken off the old plaster because of damp. Could not see any obvious sign of where coming from, actual brickwork was dry ( maybe as we had a dry spell) and thought was condensation as I had foil behind radiator. Wall has dried out completely except for an area 2 ft x 2 ft in the bottom near a corner. Have done the wall with dpc injecto cream (Sikamur) last week. Nothing against the wall outside. Went round the window outside anywhere vulnerable with mastic. Any bad pointing was raked out and repointed. Where can it be coming from? Is it coming through the bricks from the outside (the bricks do look a bit porous in places). Would painting the wall with a damp proof solution outside help at all or just seal it in. I am going nuts!

needawin, November 2011

Sounds like the damp course has been compromised, but you already know that.
You need to install a high pressure damp course to sort it out properly, as Sikamur doesn't get enough penetration.

http://www.hss.com/g/50332/Damp-Proof-Injection-Unit.html

Big Softy, May 2012