Thermal alpha particles are observed in JET during helium discharges using spectral emission in He II (n=4→3) near 4685Å following charge transfer reactions along the path of the neutral deuterium heating beams. New and reappraised He+2 /H charge transfer cross-sections are presented. The effects of cross-section energy dependence on temperatures, velocities and absolute densities deduced from thermal alpha particle charge exchange spectra are evaluated. The possibility of detecting fusion alpha particles produced at 3.5MeV and slowing-down by collisions with plasma electrons and ions using visible charge exchange spectroscopy is addressed. The spectral signature of slowing-down fusion alpha particles expected during the deuterium-tritium phase of JET is modelled and its identification against thermal alpha particle and background radiation is investigated.