Flat Roof Leaking

I live in a wimpey style home that has the flat roofs front and back. I am awaiting my builder to replace my flat roof but it seems I have a leak elsewhere also. My flat roof finishes approximately 1 foot away from the roof edge (Gable end) and I recently noticed a damp patch on the wall. After investigating in heavy rain there is water running down under the edge of the roof behind the soffits, could this be down to the flat roof leak? if not does anybody have any ideas. I have had another mate look at it and he says that all the lead is in place so is quite baffled himself. There is no sign of leaks in the house so dont know what kind of damage its doing. Please any assistance would be appreciated.

Cavity tray........lack of have a look round this forum ..loads of info on missing trays

Thanks Nige but according to my mate I dont have these in place anyway so any more comments would be appreciated.

you don`t have these in place...........exactly...........they SHOULD be in place but most aren`t because they cost money to fit.............I did 8 years on a Housing Association maintenance team ...........most of the new build was lacking proper detailing and it showed up by water getting in where trays etc should have been .........and that was 1980`s........nothing changes