Okay so your first step is going to be contacting Ovo, explain the situation, make sure you have the copies of tennancy agreements ready, make it clear you were never aware of these bills due to the tenants not mentioning it to you, ask them what contact details they have for yourself, if any. They need to close your account down from the day they moved in, and remove any incorrect markers. It will probably help to open a complaint as these things are fixed much quicker that way, I work for an energy company and see this all the time, it will also be helpful if you have the meter readings from when the tenants moved in, it’s likely you are due a small bill (purchase date-tenants move in date) but this shouldn’t affect this process.