extended warantee how many repairs do i have to accept?

we bought a tv and registered for the free option of 6 months extended warantee via sky media. after 14 months the tv deleloped a fault with the built in dvd player, we sent it for repair. it came back with dvd still faulty and now had a damaged screen! they agreed to repair a second time, they replaced the screen and replaced the dvd player again.this time the dvd was working but the new screen had a different fault so they repaired it a third time. when received back it all appeared to be working but halfway through watching a film the screen went completely black so we had sound but no picture. they have agreed to repair it a fourth time, to be collected tomorrow for 4th repair. can you tell me is there a limit on how many times i have to let them repair it before i can request or demand that they replace it? we have been without tv for a month already. family getting very annoyed.what happens if the warrantee runs out and the tv is still faulty?