Ovo changing balance

Mamaofmanybudgets

Hi, I'm sorry if this is in the wrong place. Please move or advise. 

So long story short, after an absolutely traumatic couple of years with ovo, I managed to change energy supplier. 

When I left I had a remaining balance of around £420. No problem. I made plans to pay it off but wasn't too worried. 

Paid off £150 and my balance with them suddenly became £720. 
Paid off some more again and instead of the balance going down it suddenly went up, even though they weren't supplying me. Apparently it was because they were reviewing my energy usage.  However they also told me the reason they were so accurate was because they review it, so I've no idea how it suddenly goes up everytime. 

I gave up and agreed to pay them £20 a month to keep them off my back, but it's constant calls and harassment. I stick to the £20 payments but I don't want to pay anymore because I have no idea where the money goes. 
It doesnt come off my balance and I'm scared about paying them and the balance goes up... again. 
They don't put it up when I put the £20 HOWEVER they've now shut down my online account and I cannot check balances or make payments that way. 

I just hate them 😒
I'd appreciate any advice. 
Thanks so much. 
I don't want to save the whole amount and pay because I know the balance will just change again. 
I'm a Scottish Power customer since November and it's all been plain sailing. Ive never had such a negative energy experience as I have with ovo. 

I just want to pay off the debt, which I'm able to do but just don't trust them with anything they receive 😭

born_again

Did you get a final bill from them when you switched suppliers?
Does it show the correct meter number?

FindMyWayBack

Were your readings up to date when you left them? As it is possible your owing went up when they got a proper set of readings. This can happen if your DD has not been covering your usage and debt builds up.
To give advice we need a lot more information about readings supplied etc.