A clamp is for mounting on an arm. The clamp comprises a frame, a gripping element and a moveable clamping member. The frame comprises an upright portion and a lip located at a lower end of the upright portion and extending substantially perpendicularly to the upright portion. The gripping element is connected to the frame, and is arranged to mount on the arm, and comprises a first component and a second component connected together at one end and spaced apart at the other end, the arm to be located between the first component and a second component in a hole. The moveable clamping member is mounted through a hole in the second component, and comprises a shaft and an engaging portion arranged to be moved towards the lip of the frame.