The paper deals with the simulation of neutral particle spectra obtained in JET with an array of four passive neutral particle analysers during discharges with applied stationary or modulated radiofrequency power in the ion cyclotron range of frequencies for a deuterium plasma with hydrogen minority. The distribution function of the hydrogen ions has been computed using a one-dimensional Fokker-Planck equation describing wave-particle interaction by a quasi-linear diffusion term in velocity space. Both stationary and oscillating solutions of the Fokker-Planck equation have been worked out and the computed hydrogen fluxes have been found to be in very good agreement with the measured ones. In this context, the problems arising in the deduction of the power deposition profiles in modulated discharges are discussed. The profiles of power delivered to the electrons and the bulk ions have been obtained. It is shown that no adjustable parameters need to be introduced and that the main source of error lies in the uncertainty in the knowledge of the ion density.