improvement upon previous measurements conducted in a similar fashion may be made [@muonal]. Results of these studies will be presented at a later date. Here we focus on the results from the system in its primary role of a veto detector.

Detector Stability
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Daily calibration of the veto detector with a radioactive $\gamma$-ray source is impractical: besides taking too long to calibrate all 52 modules, a high $\gamma$-ray flux inside the castle might compromise the stability of ZEPLIN–III. To probe the stability of the veto system we monitor, separately, the light transmission of the scintillators (using the fibre-coupled LED signal) and the electronic gain (by measuring the single photoelectron (SPE) response of the PMTs). In addition, we confirm the rate of coincidences with ZEPLIN–III on a daily basis, record the rate and energy spectrum of the master-mode triggers, and measure the $\gamma$-ray tagging efficiency and a number of environmental parameters which can be correlated with the data [*a posteriori*]{}. The PMT and LED calibrations are described next, followed by the methods used to synchronise the two instruments, looking in particular at how these evolved over several months of underground operation.

\[spemeasurements\]SPE calibration
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An automated daily SPE analysis examines 