I am trying to understand how the heating circuit at my daughter's place
works (without going to look)

She claims:

"When I checked what each of the meters was for this is what npower said:

I can confirm that the day rate on your meter is billed between 7.30 am and
12.30 pm. The night rate is billed between 12.30 pm, and 7.30am.
The heat boost, which is register 3 on your meter, is billed between 8.00 am
and 10.00am.
The heat units on register 4, are billed between 2.pm and 7.00pm, and the
heat units on register 5, are billed between 8pm and 10.00pm."


I m confused as to what switches the heaters on - there are 3 heater outlet
points.

Do you think she will have the ability to control the time that the circuit
is live so she can reduce the higher tariff boost times? The reply from
N-Power seems to imply a complex metering arrangement.