Hello, I am a solicitor with 20 years experience. I will try to help you with this.
I think yo should look to your lease and see what that says about responsibility for repairs to external problems like this. Very often these will be the joint responsibility of everybody in the building as all the flats have an interest in keeping the exterior of the whole building sound.
Well what does your lease say about it? How can I possibly answer your question if you don't tell me the actual situation?