And shop around for another supplier and transfer as soon as possible.
If you know the previous companies tariffs you should be able to calculate the actual amount owed, if you have the meter readings to work out usage.
Don't touch them owe me £500 since January 2019 make excuse after excuse. Seem they always have software problems sending money out. Keep saying they will call back or email nothing been chasing it now for 6 mths the phone staff always have the same banter we will chase it up and get back to you then nothing!