A non-functioning iPhone

In March I got a new iPhone 6. In April I took it on a plane. When I tried to switch it back on I got a red screen alternating with the Apple icon. According to the web this is a hardware fault and Apple will (and must) replace the phone. I took it in to Altech Autopage on 2 May and they sent it for repairs. A month later they sent it back -- unrepaired. That's it. No further communication from them. I have a very expensive dud phone. I can't approach Apple directly-- they have to. But they won't. They won't do anything at all. I posted last week but still no communication from them.