It is generally acknowledged that harmonic distortion levels on distribution networks are slowly but steadily increasing due to the proliferation of non-linear loads. While customer complaints relating to harmonic distortion levels are relatively uncommon, and measured levels of harmonic distortion are typically well within the limits specified by national regulations and international standards, the increasing trend may mean that more active management by utilities will soon be required. Effective management of harmonic distortion levels will require knowledge not only of the level of disturbance, but also of how the disturbance levels vary with time, location, and the connected load. Accepted methods of assessing harmonic levels that rely on (typically 95%) cumulative probability statistics are inadequate for this level of insight. In this paper voltage total harmonic distortion levels (THDnu) from a distribution network power quality survey are analysed in terms of variation with time, the relationship of the measured THDnu levels to system load and the physical characteristics of the monitored sites.