Recent advances in experimental measurements of magneto-optic properties of tokamak plasmas and progress in formulation of numerical algorithms for the analysis of magnetic data have allowed the self consistent determination of the current density in the JET tokamak, in ohmic and additionally heated discharges. An investigation of the numerical response of a model with finite parametrisation to the uncertainties of the discrete data available, is carried out. The error propagation is analysed for various types of discharges and results on the safety factor profile are presented.