operation of the PMTs, their gains are found to be rising on average by $\sim$0.6% per month in the first 3 months of operation. With the PMTs on the outside of the plastic shielding, it is expected that they should be affected by seasonal environmental changes in the underground laboratory. Although this effect is small, it nonetheless highlights the need for this regular calibration.

![\[speTrend\]Evolution of the mean of the SPE peak positions for all 52 veto detector PMTs over a 3 month period. The bars are the r.m.s. of the 52 means of the SPE peak positions.](speTrend.eps){width="8.0cm"}

LED calibration
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The response of the PMTs to light transmission through the plastic scintillator is measured with weekly illumination from a blue LED coupled through an independent fibre optic cable to each module. Datasets are acquired during a brief pause in the running of ZEPLIN–III used to perform maintenance operations such as filling of the LN$_{2}$ reservoir. The LED is mounted in an acrylic light guide from which 52 optical fibres emerge. These fibres are secured in recesses in the plastic scintillators at the opposite ends to the PMTs. The LED is pulsed at a frequency of approximately 30 Hz for 