A holding and sealing unit comprises a fixed member (11) for mounting on a wall and a movable member (12) pivotably connected to the fixed member. The members are provided with hooks (19, 21) for releasably retaining the side walls of a plastics material bag. The movable member (12) is movable between a closed rest setting in which a retained bag is kept closed, an open rest setting in which the bag id held open, and a transient setting in which sealing of the filled bag is carried out. Sealing is performed by a heat sealing element (24), against which the bag walls are urged by a rubber strip (23) when the movable member (12) is pressed towards the fixed member (11) and thus into the transient setting. Switching means respond to the transient setting to actuate the sealing element (24) for a preset time.