Field-cycling H-1 NMR relaxometry has been used to measure the rate of concerted double proton transfer in the hydrogen bonds of O-16 and O-18 isotopologues of benzoic acid dimers. The experiments have been conducted in the solid state at low temperature 13.3 <= T <= 80 K where the dynamics are dominated by incoherent proton tunnelling. The low temperature tunnelling rate in the O-16 isotopologue is observed to be approximately 15% faster than in the O-18 isotopologue. The difference is attributed to an isotope effect of the heavy atom framework of the benzoic acid dimer resulting from displacements of the oxygen atoms that accompany the proton transfer. Sources of systematic uncertainty have been minimized in the design of the experimental protocols and the experiments are critically appraised in formally assigning the measured differences to an effect of mass on the tunnelling dynamics.