Music Festival tickets given to wrong person.

I bought weekend camping for my friend and myself for the Download Music Festival a while ago, however, I could only get the tickets sent to my card's billing address. This is my parent's house, not mine (I'm at uni right now). The tickets arrived there fine late last week and my parents posted them up to me via Royal Mail special delivery.
No one was in my house when the postman actually arrived with the tickets so they put a little red slip through my door and the letter itself got sent to the nearest Royal Mail Delivery Office. I was planning on heading to Download mid afternoon today so I went to get them yesterday. They spend about half an hour looking before announcing they couldn't find them. They took my details and told me to come back the next day.
I went back today to find that the only thing that could have happened was that they've given my mail to a man that came in to collect his by mistake. Apparently he has a similar name and a similar address to me. Obviously I got right on it, lodging a complaint (and a request for reimbursement) with Royal Mail, then phoning Ticketmaster (whom I bought the tickets off). I was hoping they could cancel the tickets and arrange for replacements to be held in the box office but as they didn't lose them I can't even cancel the things.
Apparently the manager at the delivery office has repeatedly visited the man who has my ticket's house but no one it answering the door. I would very much like to still attend this festival if possible and I feel as though I've let my friend down as well.
I was just wondering if anyone could give me advice on who I should be phoning to try and get these tickets replaced and indeed if it is well within my rights to do so? We've tried the police already but they weren't too helpful.
Thanks in advance.