bespoke synchronisation unit clocked at 1 MHz. This sends a 32-bit digital stamp to both acquisition systems following a trigger from either detector. A number of additional methods have been implemented for redundancy. This is critical since, in case of malfunction of the synchronisation unit, the veto efficiency might be compromised for a significant part of the dataset. Firstly, an internet time server is used to synchronise the two data streams to 4 ms. It is then required that the time difference between consecutive coincident events ([*i.e.*]{}, excluding veto self triggers) agrees to within 1 ms; since the target trigger rate is only $\sim$0.4 s$^{-1}$ and all genuine coincidences have triggers sourced from ZEPLIN–III, the event time distribution proves to be a powerful selector. Finally, the summed signal of the PMT response of ZEPLIN–III is digitised into the veto data acquisition and pulse parameters must agree. Using these methods the fraction of unsynchronised events is negligible.

Detector Interaction Rates {#signalrates}
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The integral signal rate in the veto detector during science data-taking is shown in Figure \[veto\_rate\_full\]. This is the cumulative distribution of pulse sizes, given in total number of photoelectrons, as a function of threshold. Note that there may 