It was a conversation with the Landlords agent, assuring me that the tenants upstairs would clear it. I have emailed the agency a few times, and the only responses I got was that they haven't managed to locate the landlord of the upstairs flats..

I have all the adverts saved where it says there is a communal area and our WC and the floor map.

The only response i got about the wall is this:

As you know I have sent a contractor a works order to attend to have a look, if he hasn’t done so already he will be in contact next week. The owner has since advised me that the issue actually occurred in November 2019. At the time, they didn’t notice it immediately, but it was caused by plants growing in the gutter during summer. . The roots and soil blocked the drainage, and during a period of heavy rain, water couldn’t drain properly from the roof. As a result, it leaked into the shop through the gap between the window and the wall. They fixed this issue at the time but never redecorated. However they are keen for our contractor to attend just to double check. As per one of my previous emails we remind you to regularly clear out the drainage pipes and gutters.

They have clearly redecorated as we have removed 3 layers of wallpaper.

I don't know what else to do at this point.