Cement without aggregates- advice

[JohnD]

if it's not your roof, and it's the side of the neighbour's house, it isn't your concern.

Post a pic of the rainwater pipe, it might not be ideally positioned.

I think the real problem is rainwater from the roof. Tinkering with the wall is not a solution unless there is some way of fitting a verge.

 

[AmateurBodger]

Thanks for your help @JohnD . Here's a pic of the back... you can see the back of my house on the right and theirs on the left.
The moss down the back of their house maybe indicates something isn't right with their guttering/rainwater pipe as you suggest?

 

[AmateurBodger]

Also done my best to take a pic of the top of the tiles from the back of the garden in case that gives a better view - all pretty low res, but the best I could do!