I discovered a section of the internal skin of our cavity wall was totally saturated. Because of the extent of the saturation i knew it couldnt be penetrating damp from the external skin. Checked the roof and was there were no misplaced roof tiles. I was at a loss as to what was going on as there should have been no heating/plumbing pipes in the vicinity of the wall.
Into the roofspace then. Found that the overflow pipe from the cold water cistern was going down the roofspace inbetween the ceiling underneath the roof felt. It went down towards the problem area wall.
From that i had to lift a few of the roof tiles off and check under the felt between the roof joists. Couldnt see so i stuck the camera up and took a pic. With the flash on i could see that the overflow pipe just terminated about 1m from the eaves, and as this was built up, and water coming from it was pouring down into the cavity!!!
I bought the house in 2000. there was an extention built in 1998. So i am assuming that when building the extension the builder while carrying out the plumbing works has taken the original pipe off when putting on a new pvc facia to the eaves and hadnt replaced the overflow pipe!!
Normally the overflow shouldnt be needed, hence why i havent had a problem until now. But i think that the ballcock in the cold water cistern must be slighty faulty now, hence the water is sometimes overflowing into the overflow pipe and down into my f*ckin cavity wall
Im going to have to fix the overflow pipe so it projects outside and wait months for this wall to properly dry out. I'll also have to replace the ballcock.
All because a bloody builder botched up the original job and now im not sure whether the insurers will cover it

Extension should have been inspected and approved by the local council planning department, which will have certified that job was performed correctly. You may have an opportunity to seek compensation from them for a bollocksed up inspection

not without the documentation that proves he did it.

unfortunately, easiest thing to do is to just sort it out yourself.