Is voltage fluctuation in the UK electric service (i.e. supply from the DNO) a problem? How much does it fluctuate? Is there and data on this? Has anyone ever had problems with it?

Make what you will from this:

http://www.decc.gov.uk/en/cont...city/electricity.aspx


it is a large subject area.

Do mean voltage variation from one service to another ?
Or voltage variation on the same service according to load, time of day and season ?

In central London the voltage on most services seems fairly consistant at near the upper end of the permitted range.
Varies a bit with load, but not that much. Often about 250/253 volts at minimum load and about 245 at full load IME.

In rural districts it often varies far more, and often seems to be outside the permitted range.
In Dunster, a village in Somerset I have measured it as low as 210 volts on a cold evening and over 250 at other times.
At a rural public house I found an average of 220 volts with regular brief drops to under 180 when the cellar cooler started.
Had to fit a UPS for sensitive loads and a voltage regulator for the cellar cooler.
Heating oil has recently increased in price, and many people believe (usually wrongly) that electric heating is now cheaper than oil. Last winter this led to a great increase in load in some rural areas, and at least a few blown substation fuses. The transformers generally survived significant overloading, presumably due to sub-zero outside air temperatures.
It only takes a single portable heater in each home (beyond the normal load) to cause significant voltage drop, and possibly to break things.