I don't have the Maunsells other than a van BY, but all the other recent coaches from Hornby (Pullmans, Gresleys, Staniers) share the same construction. Look on the underside both ends (probably need to swing the bogies aside), and there are four transparent clips coming through the floor. Additionally midway along the body side there is a concealed clip. Ease the body side away from the chassis in the middle of the vehicle, identify the clip, and put in a thin slip of card to stop it re-engaging. Do the same either end adjacent to the clips which come through the floor. Use a narrow bladed screwdriver to disengage these clips. Repeat other side, remove body. When reassembling check that body, interior and chassis are all in the correct orientation.