Looking at the photos again and trying to work out what I am looking at from a perspective view, I note that the ridge tile(s) are missing or in very bad shape and the haunching is broken, yet the person who has fitted the presumably plastic piece has used a couple of sheets of lead to bridge the gap, which is completely wrong.
The ridge tiles need to be taken off and suitable replacement tiles using those that can be reused plus whatever is needed to cover the ridge right up to the end, and of course the haunching that is broken, removed and replaced with new cement.
Sadly it looks like a battle and extra expense getting it sorted out correctly.
You could involve Trading Standards , which I would do if it were me.