Dee Verma
hi Kevin, i have just carried out extensive repair on a job as described by yourself. it looks as you have the same problem by the fact you mention an extension. without looking it seems that you have a bridge formed by plaster when the plastering was done between the outer and inner wall of the house. to find this out you will have to remove the window where the problem is and check if it is the case. if so clean the excess plaster which has been applied causing the bridge and then insulate the cavity using DPC and cavity closer this should have been done in the first case when it was being built and the refit the window. this should solve your problem.
