Night-time electricity tariffs
I've lived in a few places with 'Economy 7' (also known as 'white meter' in general parlance). Basically an electro-mechanical clock clunked over to record the cheaper rates at 11:30pm-12:30am (yes, 1 whole hour), then back again to normal rate for the next hour. Then 1:30am-7:30am was cheap rate again.
The good thing was that the power sockets were also on the cheap rate, so you could run the dishwasher, washing machine and the tumble dryer on the cheaper rate too. Usually by setting the delay start timer on the appliance, or by using a timeswitch adapter.
The heating in the flats were electric storage heaters (Creda, Dimplex). And the water heater was an immersion element type.
There was another type of cheap tariff electricity which replaced the Economy 7 version, called 'Total Heating with Total Control' - but it was a total con.
In this version, only the storage heaters, immersion heater, and electric fire were on the cheap rate. The power sockets were on the normal dearer rate, and the storage heaters had booster convector heaters built in to them too - which were wired into the normal dearer mains.