Which part of the lead work would it be getting under.? I was looking at the white sealant stuff on the left of pic 1 and was thinking maybe holes have formed and rain getting in that wayl. In the bathroom ceiling its basically the same position of the leak.
OP,
The abutment flashing is a mess.
Briefly, you would be better off with cover flashing and soakers.
But you would first need to remove all the visible flashing, & lift the abutment tiles to examine the conditions below the tiles.
I assume its a dormer with two valleys higher up - what condition are the valleys in, and whats the state of the far side of the dormer at the other abutment?
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