Page 1 of pc world, advice needed

Hiya all,
i purchased a laptop from pc world in September and in December it developed a fault, so i tried to return it but they said that cos it was passed 28 days they wont exchange but will repair it. So I agreed to this and had it repaired, but now the same fault has occured and i feel this is not good enough, the same fault happening twice in 6 months is not good.
Does anyone know if I am entitled to exchange for a different laptop or a full refund. As I feel that if it has happened twice it will happen again and i dont want the warranty to run out and i cant do anything about it.

please help me.

Steve.