Okay, easy. Take a friend in with you, or family, and ask to see the manager. Now, if they refuse to let you see the manager, tell them this.
'If you do not bring the manager here right now, I will call up the better business bureau and have this store invetigated for you all violating your own policies. So, unless you want to lose your job and get taken to court, bring me the manager right now.'
You see, the Better Business Bureau specializes in cases like this, in which employees and managers start violating rules and laws for retail. If they will not refund faulty items in the timespawn of their own warranty, the store CAN be shut down by the BBB. In fact, the BBB is capable of taking any employees or managers, or both, that violate policies to court. So, just remember about the BBB, since if Tesco doesn't follow their own policies, just leave. Call up the BBB, where you and your witness, friend or family, can report what happened.
All it takes is one complaint to the BBB, and you could get Tesco's workers in a LOT of trouble. Trouble they brought on themselves.