Gd is two-fold. Firstly, the states populated in $^{158}$Gd by the neutron capture decay with the emission of up to 8 MeV distributed amongst typically 3–4 $\gamma$-rays rather than the single 2.2 MeV $\gamma$-ray that follows radiative capture on H. The inclusion of Gd, thus, increases significantly the neutron tagging efficiency. Secondly, the mean capture time is reduced relative to that on H. This allows adoption of a shorter coincidence window resulting in reduced data volume and a lower accidental coincidence rate. The Gd, in the form of Gd$_{2}$O$_{3}$, is mixed into an epoxy and set into slots in the polypropylene sections, with slot pitch and width 10 mm and 2 mm, respectively. The veto detector and the ZEPLIN–III instrument stand on a 30 cm thick polypropylene base and are enclosed within a 20 cm Pb castle. A single blue LED transmits light through 52 optical fibres to the far end of each of the veto detector modules for weekly monitoring of the scintillator/PMT response. Each PMT output is digitised with 14-bit resolution at 100 MS/s for a waveform duration of 320 $\mu$s.

The operation of the veto detector allows for a combination of ‘slave’ and ‘master’ modes. In [*slave*]{} 