There are many factors which determine the used value: hours of use, condition of bezel, scratches on screen, dead pixels, edge bleeds etc etc. So cannot give a value.
However, very surprised that you paid £1100 for a Samsung 40" A5 series model - £700 would be nearer the mark.
At a rough guess you might achieve £400 for an unblemished model.
Sorry I can't be more helpful!