apple provided fake screen on warranty phone

I purchased a 5S for my daughter from apple west quay store in Southampton in Sept 14. After a few months the microphone broke. My daughter returned to the store and they replaced without issue. The problem started a few weeks ago, my daughter dropped her phone so she returned to the same West Quay store for a screen repair. The store refused to repair as they said it had a fake screen fitted. We had not replaced the screen so the warranty had obviously been a repair before we had the phone given as a warranty. Apple are refusing to repair even though we received the fake screen from them. Does anyone have any ideas? thanks