Leaking roof valley

[Ubtree]

One of the valleys on my roof which was leaking has been repointed, but the leak continues.

I attach 2 photos - one showing the main run of the valley, and a second showing a close-up of the pointing at the bottom of the valley.

I have 2 possible theories as to what has gone wrong:

1. A YouTube video suggests that the mortar should not cover the ends of the joints in the tiles (as illustrated in the attached screenprint). The pointing does cover over these joints.

2. The mortar at the bottom of the valley (as shown in the second photo) was in good condition, and wasn't replaced. On each side, this mortar completely fills the gap between the pointing and the lead upstand. Does there need to be a gap, to allow any water trapped between the pointing and the lead upstands to escape?

Do either of these suggestions hold water , and are there any other likely culprits?

 

[datarebal]

Tiles don't over lap the lead enough and water will get drawn over the lead turn up by capillary .
Basically the lead to narrow. And the tiles shouldn't be bedded directly into lead .

 

[^woody^]

The lead is turned up, so if that is the same up the valley, then it should catch any water going through the mortar.

If that valley has been like that for years and things have been dry, and the only work done is the repointing, then I would suggest that the pointing was not the problem in the first place

 

[datarebal]

The lead is turned up, so if that is the same up the valley, then it should catch any water going through the mortar.

The problem arises when the mortar bridges the turn up , judging by the overlap this is very likely.
That aside, its incorrect to bed directly on to lead