In the JET tokamak the perpendicular velocity distribution function of Ion Cyclotron Resonance Frequency (ICRF) heated ions is inferred in the energy range 0.3≤63E(MeV)≤1.5 from Neutral Particle Analyser (NPA) measurements of the emitted high energy atomic flux. The measurement is integrated along a vertical line-of-sight through the plasma centre, with a finite viewing solid angle. We present a method to deduce the ICRF heated minority ion density in the plasma centre taking into full consideration the geometry of the measurement. We verify our results against independent measurements of the fast ion density and perpendicular energy content, and we obtain quantitative agreement. Our results further validate the model of impurity-induced neutralisation, which was developed to interpret the high energy atomic flux measurements in JET. They give access to a fundamental property of the ICRF-heated ion distribution function, and provide improved tools for understanding the behaviour of fast ion populations in forthcoming fusion experiments.