@alowe You have a Comfort Plus system, which is fairly unique to SP. The storage heaters get an extra boost when the weather conditions are cold in the day, this is the third rate shown as Control after day and night.
You can read a lot about it by putting 'Comfort Plus' in the Community help search at the top of this page.
People often try to switch away from comfort plus to a conventional Economy 7 tariff, but it seems that the system is usually hard wired and cannot be changed without an electrician.
In favour of comfort plus it gives cheaper electricity than a standard single rate system and many people find it economical.
As the control rate has skyrocketed (100% + increase), I have seen my winter heating costs shoot up by around £400 since last year. I'll definitely look into other suppliers off-peak options before next winter.