JET plasmas show fluctuations from a large variety of MHD instabilities. A combination of spectral analysis, mode number analysis and determination of mode radial position has the potential as a diagnostic for the plasma q-profile. Mode frequencies and mode numbers are obtained from spectral and phase analysis from fast magnetic pick-up signals. Cross-correlation of magnetic and multi-array signals from ECE, SXR and reflectometer systems provide the mode localisation. The present status of the diagnostic systems used for fluctuation analysis is summarised, with particular emphasis on the fast magnetic pick-up system. A new data collection system allows the magnetic signals to be recorded at 2MHz for up to 32s in each pulse. An example of q-profile validation from the observation of Alfven cascades and a snake in an optimised shear discharge is given.