We have a long-established interest in the structural role of heavy-metal, lone-pair cations such as Pb in glasses. PbO can be incorporated to very high concentrations in oxide glasses, where it must constitute part of the network by forming Pb-O-Pb links. We have used roller quenching to form lead silicate glasses, xPbO¿(1-x)SiO2, to x = 0.83. Previous studies have suggested the formation of edge-shared dimeric units [Pb2O4] bonded into the silicate network at high values of x. We wish to confirm their presence by measuring the Pb-Pb distance using neutron and x-ray diffraction. At high values of x, there is NMR evidence that the silicate ¿network¿ consists almost entirely of isolated [SiO4] and [Si2O7] units and we also wish to confirm this by attempting to resolve the Si-Si distance by the X-ray/neutron difference method.