As others have said you need a tariff that's hourly, be warned though people got bitten with this when prices soared to many pounds per kW/h, it's not always this low.
The most common tariff that offers these prices is capped at 100p/kWh (about 3-4x higher than the standard price). I've been on it for about 3 months and I never saw anything higher than the standard price.
