Water Ingress Into Loft Space And Into Room Below

[Smudge01]

Rainwater is entering the attic (bottom left of this roof hatch, just about where the slates end and the valley starts). Above the hatch and the rest of the roof the slates all look good, nothing looks wrong, so (I'm assuming), the water must be getting in along the left hand edge of the upstand of the hatch and behind the slates. But this roof has been like this for 50 years or more, why is this suddenly happening now?

Ordinarily shouldn't the lead sit on top of the slates?

What would be the best way to try and diagnose this fault. Most of the slates have been glued into place around the hatch so I can't just slide a few out and have a look, I'd have to smash them to get them out?

Any help or pointers would be appreciated before I get my hammer out.

Thank you in advance.

 

[datarebal]

Side flashings are wrong side of front apron as is the last soaker. should be over not under

 

[Smudge01]

Thanks for your reply.

Sorry but I'm not sure exactly what you mean, are you saying the flashing down the side should sit on top of the slates?

 

[datarebal]

The flashing type is fine with soakers under the slates.
Apron flashing may have slipped out in the past and been slipped back incorrectly.
It isnt well made but that aside the side flashing and last soaker should run onto it not under

 

[Smudge01]

Got it, thank you.