Dell are not repairing damaged laptop.

We bought a Dell computer January 2014. Asides from other problems In March 2015 (within warranty) the screen started flickering. Dell rejected cover. We then GRUDGLINGLY paid their quote for repair. Four weeks later they have not repaired this!! My daughter travels between varsity and home. We have therefore booked repairs in advance. They cancel every time with various excuses. Thier cell messages and emails are not coherent and honestly their systems are badly managed.