If that flat did the damage it should be their responsibility. If it was another car crashing into yours the driver of the car that did the damage could not tell you it was there fault. Well they could but if there were witnesses they wouldn’t get away with it. So do the same. Take some pictures of the damage and tell them their flat is causing damage to yours. Tell them they need to fix their problem asap and I think what has to be done is you report this to your insurance then your insurance contacts their insurance and it is sorted out. If you are a leaseholder, your policy for your flat is through the block policy for the building so you need to call them up. Probably is the Zurich one tel them the problem and the flat number that caused it then they will investigate and they will either send a contractor to get it fixed or ask you to find one and then pay you for the amount you have to pay the damage. If worse came to worse, you would have to tell the Council you would take them to court to get the damage fixed. That should get them thinking. We had water damage once from upstairs flat. The block insurance paid for the damage. Didn’t approach the council, just contacted the insurance.