The Retarding Field Analyser (REA) is essentially the only practical tool for the measurement of ion energies parallel to the total magnetic field in the tokamak scrape-off layer plasma. One such device has recently been successfully employed at JET . Its bi-directional nature allows both measurements of ion temperature and plasma flow velocities. The latter are computed by collecting the ion saturation flux to negatively biased entrance slit plates which are set back inside a boron nitride protective housing. When comparing the REA slit currents with those measured by a turbulent transport probe (TTP), the RFA fluxes are found to be a factor 4 to 5 lower for very similar plasma conditions. Using Paiticle-in-Cell (PIC) simulations, this contribution demonstrates unambiguously and quantitatively, that the REA flux attenuation is induced by the magnetic pre-sheath formed in the orifice in front of the slit plates.