This paper describes measurements of the sawtooth instability in JET, in which the instability wave function is shown to extend to the edge where it is measured using magnetic coils. The numerous magnetic probes in JET allow the time evolution of the (n = 0, 1, 2, 3) toroidal Fourier components to be analysed. The n = 1 magnetic component is measured to be similar to the m = 1 soft X-ray centroid motion. This fact indicates the potential of edge signals in retrieving the poloidal mode spectrum of the q = m/n = l surface. The spectrum evolution of the instability is compared for normal sawteeth (NST) and quasi-stabilised "monster" sawteeth (MST). The spectrum is slowly decreasing with n for NST and all the components belong to one ballooning like deformation, whereas MST show a large n = 1 kink-like motion with small and independent accompanying higher n modes. Important equilibrium changes are measured to occur already during the growth of the instability and the growth rate is much faster than exponential, both facts implying the non linear nature of the instability growth. Parametric dependence of growthrates, amplitudes, toroidal spectrum shape, etc., are studied to characterize the NST and MST instability.