1. A metal object detecting apparatus comprising, (i) a transmitter for generating a time varying primary magnetic field within a surveillance volume, the magnetic field having a non-zero magnitude throughout substantially all of the surveillance volume and a resultant magnetic field direction which varies along any substantially linear path through the surveillance volume such that at three locations along said path the resultant magnetic field points in three mutually substantially orthogonal directions, (ii) a detector for measuring a secondary magnetic field at a plurality of positions as a function of time due to the presence of a metal object within the surveillance volume as it passes a plurality of measurement points there-through, the detector being arranged to substantially reject the primary magnetic field, and (iii) a processor for determining from the measured secondary magnetic fields a track through the surveillance volume comprising a plurality of locations of the metal object within the surveillance volume and a magnetic moment thereof at each location, the processor being adapted in use to derive there-from a magnetic signature that is characteristic of the metal object and independent of the orientation and track of the metal object through the surveillance volume.