PC World/Currys lost my Laptop

TeaLady5

I purchased a laptop from PC world/Currys and it went faulty within the 12 months warranty. They collected the laptop from my house, and have now had the laptop over 21 days, I have been constantly phoning them (including being on the phone waiting for them to talk to me for over 2 hours at a time, ) and they have said that still can't find it, but they are sending an email to the repairs team that I had called to chase up the computer repair. But now they say they need an extra 7 days to fix it but how can they when they have lost it?
How can I get a replacement or a refund?
thanks