Time to request a voltage recorder to be installed then :-)
Load has traditionally only gone up rather than down, so depending on how far away from the transformer you are you may see more (or less) of a dip
- the further away you are, the bigger the dips are likely to be.
IIRC they have to provide a "substantive" (written) reply within 10 days, and should report back the results of the voltage recorder.
Try these (watch the wrap)