On Sunday, 15 September 2019 13:57:36 UTC+1, Robin wrote:
Probably an idiot-boy question: why would the meter need to be replaced
for E7 or E10 when it is labelled "Multi Rate Single Phase Watt Meter"?

A very sensible question.

The existing meter needs an external timeswitch or teleswitch to (a) change the meter from rate 1 to rate 2; and (b) to connect the heating load only at off-peak times. It only has L and N supply, and single L and N for load.

More modern meters include their own timeswitch or radioteleswitch (or smart meter equivalent) and have separate Live outputs for 24-hour *and* off-peak, so there is no need for any timeswitch or contactor to control the heating load, and the heating load switching cannot get out of synch with the meter rate changing.

This is also a bona fide application of smart metering - the heating load charging periods can be moved to accommodate varying demand on the grid.

It also makes for a much neater installation with usually only 1 henley block needed to split the neutral (and not even that if a dual tariff board is fitted)

Owain