A question for you roofing experts.
On this flat roof, all the rainwater is directed to one side and the front. That has a lobby beneath it, so a sloping slate roof down to above the door to the roof terrace - at half landing height below the flat roof.
How would the felt roof be configured where it meets the sloping slate roof? (Top of that slate roof is level with the felt) My guess is this is where the problem is, and to some extent always was.
One side of the slate roof is a brick parapet party wall, with lead flashing. The other side lead covered ply.