Neutral particle analyser (NPA) measurements of emitted hydrogen flux, integrated along a chord through the plasma centre, have been used to infer a perpendicular tail temperature T_⊥ of minority ion cyclotron range of frequencies (ICRF) heated protons in the Joint European Torus (JET). A relation is established between T_⊥ and the perpendicular proton temperature T_⊥(0) at the position of maximum power deposition. At energies E >> T_⊥(0), the value of T_⊥ inferred from the NPA is nearly equal to T_⊥(0). In the opposite limit E << T_⊥(0), it is shown that T_⊥ << T_⊥(0). Applying this model to NPA measurements from a JET pulse with varying ICRF power P_RF, it is found that the scaling of T_⊥(0) with P_RF in the 2 to 10 MW range is almost linear, as expected. Moreover, calculated values of the ICRF heated proton energy content, based on deduced values of T_⊥(0), agree with diamagnetic measurements. The NPA measurements thus give access to a fundamental property of the heated proton energy distribution, which plays a crucial role in sawtooth stabilization