I have octopus intelligent plus ohme charger.
The ohme app is confusing to use to start off with. The main thing to remember is you are always adding the percentage it says. So best thing to do is check your already charged percentage and add the remaining charge you want on ohme.
Example
You want car charged to 80% by morning. Your car is already charged to 35%. Set ohme app to charge 45% by 5.30am (I found that you will actually need to set it too 48% ish because it uses some extra energy for the charge process, generally more the colder it is).
As for the in home display mine always shows the energy used but at the normal tariff not the overnight tariff but when the bill comes it is always the correct tariff rates that I pay. So the display shows I used £10 to charge the car overnight but I only used and get billed £2.50 .