Studies of the impurity behaviour in large tokamak plasmas during additional heating is of fundamental interest because of the resulting increase in radiated power and ion dilution. In particular, mid- and high-Z impurities can contribute to the average Zeff during high power Radio Frequency (RF) pulses. Furthermore, the VUV radiation emitted by the metallic impurities at the plasma edge affect the plasma density limit and the quality of the RFinduced H-modes. The impurity release from the screens of the RF antenna may significantly limit the heating effectiveness. Since the metallic impurity radiation increases with the RF power and pulse duration, it is specially relevant to the assessment of the ITER-Like Antenna (ILA) in JET, where powers of up to 7.2MW are expected. This is facilitated by the provision of metallic impurity densities for large numbers of discharges and, to this end, a method for determining the densities in steady-state plasmas using passive spectroscopy is presented. In the first instance, this method has been applied to Ni, the commonest metallic impurity observed in JET.