I have moisture ingress on the interior of both Gable walls. Last year the moisture started on 1 wall, I had a builder around who said it was cracked rendering which i then paid to have fixed. It didnt solve the problem and in the meantime moisture started to ingress on the opposite Gable wall which is attached to another house so couldnt be rendering this time. I had a 2nd guy around who said it was due to cracks in the parapet walls which he then sealed with a PVC coating.

The problem is only getting worse and i fear it is to do with the flashing on the roof.

I have access to the roof via a roof terrace so very easy to access without ladders/scaffolding. The roof is a pitched slate roof.