I've just started with Outfox The Market. Electric cost is 9.7062p/kWh, day and night. There is no daily rate (aka standing charge).
I was with ExtraEnergy, whose prices were rising to 16.437p/kWh day, 9.914p/kWh night.
As somebody said, if you have a smart meter you might have day and night prices already, and it probably doesn't need an automatic switch.
That is really cheap. 9.7p and no standing charge. . . I shall have to look that up. [EDIT doesn't look so good once you factor in £75.10 a year to join the "Energy Hero Club" (a standing charge in all but name?), and 11.4p in Colchester
Having said all that, there is quite a LOT to like about the offer this company makes mumble mumble mumble]
Basically, all electric heaters are equally efficient at converting electricity to heat, but the better the lagging on your cylinder, the more of that heat will actually be used by you for hot water. Heat escaping the cylinder will still heat your home, but this is a waste during the summer.
Going onto an economy 7 tariff will probably give you more expensive day rate electricity, so the question is really how much of your demand for electricity you can switch to the night. Storing the heat energy (e.g. by keeping water in an insulated cylinder) so that you buy it when it is cheap and use it when it is more expensive can work well (and a wet central heating system from a large electric thermal store
could be ideal), but if you are on economy 7 and use a tumble dryer during the day, take electric showers during the day and find that your storage heaters actually struggle to store the heat from the previous night until 7 o'clock in the evening when you actually want it, then economy 7 (or economy 10 may be worth looking into) may not be for you.
In the meantime, you will have less heat loss using ONLY the boost as you are storing less hot water, so if you are not on Economy 7 tariff, then just using the boost may be a good idea. Probably worth heating the cylinder through once a week though using the other switch, just to keep things hygienic.