@GeorgieHow often have you been giving SP meter readings? If you haven't given them any for a while they'll be billing you and upping your DD based on estimated readings - the bills will say so - and their estimates are usually wildly inaccurate. They like people to be in credit, you see.
If that's the case you should give them up to date readings and that should correct your balance. You can then ask for a refund of anything over-charged.
If your bills DON'T say they're estimated, you need to call SP and get them down to your place, rip out that smart meter.. They don't look at this forum unless someone is rude or swears so unless you ring them they won't know.