Leaking Lead Box Gutter? Desmopol.

[JamesP]

I will be replacing a broken / cracked, stepped fixed double glazed unit on a very shallow conservatory roof to see if that resolves the issue first.

The above feeds into the lead lined box gutter where water is appearing  inside the room where the box gutter sits.

I cannot see any obvious cracks in the lead. The pointing on the wall is also very old.

 

Would a Desmopol type product be suitable to cover the lead / brickwork?

Any input appreciated.