Refusal to replace faulty phone

I have had an Autopage Account since at least 1999. I received upgrade Samsung Galaxy S6 via Courier on 10 December 2015. This Phone faulty in that it kept switching itself off and on, more often just switching off even during phone calls. First opportunity to take in the phone was on 17 December 2015 - within the required seven days. The phone came back on 14 January 2016 still with its original defect. I have phoned Autopage five times and have returned the phone to the Kenilworth Store at Kenilworth Centre. The phone has been booked in for \repairs"but a replacement unit is what is required and what I am legally entitled to receive but Autopage is refusing to do so. Having been a Client for 16 years"