Very sorry to read of your problems, comloulou, did not notice your question until today. I cannot add to the substance of answers you have already received except that "expert" opinions can be seriously unreliable. Which part of the UK your flat is can in fact be significant because, if I am not mistaken, laws on this sort of thing vary somewhat. It does sound as if it might be worth getting copies of all the different reports together and, with your neighbours, decide whether to challenge some of the verdicts - it is entirely possible that one bad verdict will reflect on the entire building from top to bottom. The suggestion, that the surveyor who issued the verdict which troubles you should be asked to reconsider, is a good one.