I brought one of my 840s from a local store (end is world) and I took it back explaining this issue after having it for 2 and a half months and they changed it for a 850 and I paid a extra £3 for it. no problems at all
Mine was bought about 18 months ago from OCUK, so they told me to contact Samsung.
Good to hear that "world" didn't mess about. That's good service.