Localised and intense ion Cyclotron Radio Frequency (ICRH) heating of two-ion species plasmas in the Joint European Torus (JET) tokamak produces minority ion distributions having characteristic energies in the multi-MeV range, similar to that of fusion α-particles. For typical values of the peloidal magnetic field, the size of the particle orbits can exceed significantly the width of the RF deposition profile. In the presence of a negative radial gradient in the ion-electron slowing-down time, the finite orbit-width effect gives rise to a significant reduction in the calculated energy content of the minority particles. A model is described which has been compared with RF heating data from the JET tokamak with both hydrogen and 3He minority ion species. The model is capable of explaining previously observed discrepencies between measured fast ion energy contents and those which have been calculated using a zero orbit-width model. The largest corrections to the global energy content of the fast ions are ≈56% and are found for low-current hydrogen minority cases in which the axial ion-electron slowing-down time is long (≈1.5s). Within the experimental scatter, we have been able to place a upper limit of Dfast ≤ 0.18 m2 s-1 on the fast-ion diffusion coefficient for any non-classical energy-loss processes.