Two reciprocating probe systems, at the same poloidal position at the top of the JET torus but toroidally separated by 180degrees, have been used to measure parallel flow in the scrape-off layer (SOL) of lower single-null, diverted plasmas. One system uses the entrance slit plates of a retarding field analyser to record upstream and downstream flux densities, whilst the second employs two pins of a nine-pin turbulent transport probe. Measurements have been made for both forward and reversed toroidal field directions. The results from both probe systems are similar. In the forward field direction, that is with the ion B x del(B) over right arrow drift direction downwards towards the divertor, a strong parallel flow is measured at the top of the machine in the direction from the outer to the inner divertor. The flow generally has a low value, Mach number M similar to 0.2, close to the separatrix, but rises in the region of high magnetic shear close to the separatrix to a maximum of M similar to 0.5 some 20 mm outside the separatrix. In contrast, for a reversed field, the measured flow is small (close to zero) throughout much of the SOL but rises near the separatrix to a value equal in both magnitude and direction to that observed in the forward field. There is thus some symmetry in the flow with respect to field reversal but with a symmetry axis given by a positive offset of around M similar to 0.2. This paper presents simulations using the EDGE2D/Nimbus code, which predicts very low values of parallel flow Mach number near the probe position. The possibility of impurities released from the probe surfaces increasing the flow velocity is explored using the code.