although from what I've read the way Ovo apply the correct cheap rate is charge you at your standard tarrif rate and then credit you back the difference from the 7p per Kwh at the end of the month, which seems a bit of a roundabout way of doing it although I expect there is a benefit to them doing it that way!
Yes, (conspiracy time) the benefit to them is they routinely 'miscalculate' your credit and you need to watch your bills like a hawk to make sure you actually get the right credit each month, but the month it's applied to isn't the month the credit was generated in so it can be confusing and messy to keep track of and make sure you're not losing out.