This does happen. But at the end of the day if your doing something your not competent at doing then you've got no cause for complaint if the customer isn't happy. Regardless if you told them your a tiler or not.
I know how to do electrics, i worked on 3 phase generators whilst in the Army. Still got a sparks out to wire up the UFH i'm on at the moment. Because i'm not qualified and have no come back if something does go wrong.
I wonder if your insurance covers you for tiling work? It all does sound a little suspect if you've done all the work and they've waited until your finished before picking fault. But first an foremost they should ask you to rectify. But in the same respect, you've come here for help and since you haven't had the warmest of welcomes you've not been back despite everyone still offering to help. That in itself seems a little suspect. Maybe there's a lot more to do. Who knows.
