Apple repair took off my glass screen protector and didn't even fix my phone.

So I had my iPhone sent in to be fixed as 2 of the buttons had broke and I had warranty on the iPhone so I took it in to be repaired. They said it was cheaper to replace it then repair it and asked for £360 for a new one, I had just bought it off a friend so I wasn't going to pay such a price so I had it returned. However when it came back, it was missing the glass screen protector I had on it. Now, this screen protector was £20 and when you take it off you can't put it back on (just like most glass protectors) so I'm outraged! They didn't repair my iPhone even though it was under warranty and now I'm £20 down because they decided to take it off and throw it away. I still have the box and receipt for the screen protector so I have proof of ownership, what should I do to get my money back?