One option, if you are taking over a tenanted property, or indeed are a tenant is to move your Gas and Leki to 'Spark'
It's a supplier used by landlords.
They are no cheaper than the rest, they are more of a broking service than an actual supplier, I assume they take a cut from the big supplier such as NPower and EDF etc
The advantage is they are totally geared up to deal with properties that change hands, switch meters, switch suppliers, etc.
So they dont seem to be phased by odd meter readings, units that dont match the central database, or money not owed by the current resident.