Defective Tablet - \Repaired\" - still not working"

I bought a Blaupunkt 10\ Tablet in december 2014 from Matrix in Margate. Beginning August 2015, the Tablet started showing strange lines over the screen. I Brought it in on 05/09/2015, and was told they would have to send it for repairs. It arrived back on 09/10/2015, supposedly \"repaired\". not even 2 months later I was sitting with the same problem. It was sent away AGAIN on 12/12/2015, and this time it took almost 9 WEEKS before it was returned on 05/02/2015, apparently \"Fixed\". not even 3 weeks after receiving it back, THE EXACT SAME PROBLEM started again. After getting in touch with Jon from the Margate branch, I was told it would need to be sent away AGAIN, but very cheekily added \"They won't guarantee this\". If I send it away again, I will need to wait heaven knows how long for it to be \"Repaired\", and will just develop the same issue within a few weeks. How would I go about Requesting a Refund? this product is obviously defective, and Jon has successfully strung me along long enough for the warranty to expire. He has now stopped responding to my mails. "