Samsung YP Z5 Faulty second opinion required

My YP-Z5 developed a fault whereby it cannot be switched off it just goes into standby mode, only way to turn it off is to use reset button.Sent it off to Samsung authorized repair agent came back working fine. Fault reoccured a week later it has so far been back four times (recorded delivery) and once I have even paid £17.00 to get it back as no fault found and have just received another letter saying the same. I spoke to Samsung and refused to pay they authorised the repair agent to return it and say they can do no more and they can only act if I get a second opinion. Can anyone advise me of any reputable companies that carry out this sort of thing as I wish to carry this through as a matter of principle. I would be pulling my hair out by now if I had any.

Regards

Tuneless