Changing Utilities supplier.

I have changed my supplier from EDF to Octopus from today.
This is mainly because, to me, EDF’s bill were gobbledegook.
I submitted readings yesterday to Octopus as requested and today received a Welcome email.
They say that any refund owed to me from EDF can take from 8 weeks to 8 months to be repaid.
Do you suppose if I owe them they will wait that long for repayment?
I doubt it.