Flat Roof Leak - how easy a fix??

[ShabbaRanks468]

Hello I’m after some advice, I keep getting a leak in certain weather conditions, usually when there’s a wind. I would imagine the wind blows rain into a gap I’ve found in my roof felt. It’s definitely big enough for water to be getting in (see pictures). The gap is in a corner of the roof where my flat roof meets next doors at a right angle. Just wondering if I’d get away with nailing it back down flat in the 2 spots pictured (either with CT1 or screwed down) and plastering it with flexacryl / something like and having some flashing put over the top . Or if anyone has any other suggestions I’m open to ideas, cheers