An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: FIG. 1 is a vertical section through the mechanical part of the machine, FIG. 2 is a plan view of the machine shown in FIG. 1, FIG. 3 is a horizontal section taken at III--III of FIG. 1, FIG. 4 is a circuit diagram of a photocell and detector circuit, FIG. 5 is a