First experimental results are reported of anisotropic slowing-down features observed in JET helium beam fuelling experiments. Two independent observation ports, one with a view perpendicular to the magnetic field in the centre of the plasma, and a second multi-chord viewing arrangement, approximately tangential to the toroidal field, provide radially and temporally resolved information on the velocity distribution function comprising the populations of both fast and thermalised alpha particles. The fuelling process is characterised by a change-over from a distinctly non-Maxwellian distribution function to a dominantly Maxwellian distribution and also by a broadening of the deduced fast ion density radial profile. The fast particle component in the observed composite charge exchange spectrum is found to be in excellent agreement with predictions which are based on anisotropic velocity distribution functions obtained from the analytical solution of the neutral injection Fokker Planck equation. Absolute particle densities of both slowing-down and thermalised alpha particles are determined and compared to densities and particle numbers expected from beam current and fuelling time. Thus quantitative data consistency is established for alpha particles from thermal energies up to the 3He0 injection energy of 150keV. Sawtooth oscillations in both thermal and non-thermal alpha particle densities are found in low-power, low-temperature plasmas with no additional RF heating and with sawteeth periods comparable to the slowing-down time. Signal-to-noise levels in the measurement of fast alpha particles in the JET helium fuelling campaign are extrapolated to thermo-nuclear fusion alpha particle density levels expected for the D-T phase of JET. It is shown that beam penetration and not competing continuum radiation is a major constraint, and that acceptable (hydrogen or helium ) neutral beam power and energy requirements promise a feasible ex alpha particle diagnosis in the core of next-step devices such as ITER.