  This article describes the Monte Carlo simulation used to interpret the
measurement of the muon-induced neutron flux in the Boulby Underground
Laboratory (North Yorkshire, UK), recently performed using a large scintillator
veto deployed around the ZEPLIN-II WIMP detector. Version 8.2 of the GEANT4
toolkit was used after relevant benchmarking and validation of neutron
production models. In the direct comparison between Monte Carlo and
experimental data, we find that the simulation produces a 1.8 times higher
neutron rate, which we interpret as over-production in lead by GEANT4. The
dominance of this material in neutron production allows us to estimate the
absolute neutron yield in lead as (1.31 +/- 0.06) x 10^(-3)
neutrons/muon/(g/cm^2) for a mean muon energy of 260 GeV. Simulated nuclear
recoils due to muon-induced neutrons in the ZEPLIN-II target volume (~1 year
exposure) showed that, although a small rate of events is expected from this
source of background in the energy range of interest for dark matter searches,
no event survives an anti-coincidence cut with the veto.
