Nick wrote:

The other concern is how does the electricity meter measure power drawn by a
device with a low power factor ? - does it over or under read ? Also does
this vary between the older eddy current spinning discs and the newer
flashing LED meters with some other method of measuring power.

Domestic meters are designed to read true energy/power only. They are
carefully designed to perform this calculation.

Power = Voltage x Current. The meters are designed in such a way as to
measure the instantaneous voltage and instantaneous current, multiply
them, and then record the result.

In the case of mechanical meters, there are some cleverly arranged
electromagnets - one produces a field proportional to voltage, and one
proportional to the current. They are arranged so that one induces a
current in the disk, such that the current flow is at right angles to
the other magnet. The result is a torque on the disk which is
proportional to the instantaneous voltage and instantaneous current.
Finally, there is a permanent magnet which causes eddy current braking
on the disk, making the disk rotation speed proportional to the torque
acting on it.

Electronic meters use analogue-to-digital converter chips to measure
voltage and current at several 10s of kHz (using appropriate
shunts/potential dividers). An integrated microprocessor performs the
multiplications digitally and adds up the total energy. It also triggers
an LED every time a certain quantity is totted up.

The mechanical ones tend to lose accuracy outside of their nominal
range. Friction leads to inaccuracy at low and high ends, and extremely
poor power factor can also degrade accuracy.

The electronic meters design is such that they should accurately record
at all loads, and all power factors, including the nasty waveforms
produced by electronic equipment. I'd expect them to be better than the
mechanical meters in this regard.