A similar thing happened on our last CW flight from NAS to LHR. The plane had already flown from GCN to NAS where we boarded. CW was packed with 'staff passengers' that had been moved up by the crew. We (and other CW passengers boarding at NAS) had to ask these passengers to vacate our seats. They did, but they took the CW bedding with them and left the seats covered in litter - my wife's seat left with molten chocolate on it which the crew could not remove so they gave her an extra blanket to sit on.