A new Magnetic Diagnostics system has been designed for the 2005 JET experimental campaigns onward. The system, which is integrated into the existing Magnetic Diagnostics, consists of in-vessel and ex-vessel sensors, and is intended to provide an improvement in the accuracy and reliability of the present equilibrium reconstruction and iron modelling capabilities. In this paper the ex-vessel system is described, which consists of an array of pick-up coils, Hall probes and flux-loops probes to be installed in the proximity of the upper horizontal iron limbs and inside the iron collar. The sensors are designed to produce an absolute measurement of the magnetic field configuration in the vicinity of the magnetic structure of the machine, which is considered valuable information for the tuning of the axisymmetric models used for plasma equilibrium reconstruction. On the basis of recent results obtained with Hall probe prototypes on JET, the choice of type and location of probes, together with other design and manufacturing issues of the ex-Vessel magnetic diagnostics are presented and discussed.