A simple procedure has been devised to extend the analysis of external magnetic measurements, using a freeboundary equilibrium solver with the additional constraint of diamagnetic measurement. The paper discusses the application of the method to ohmically and auxiliarily heated JET plasmas where pressure anisotropy could occur. It is shown that approximate relations commonly in use for a fast calculation of poloidal beta and internal inductance are not accurate in the case of JET. A better approximation for isotropic pressure is given.