Previous bill overcharged, final bill minus amount.

[#1]

Transferred to another provider.
Contract with Plusnet due to finish on 5th of January.
December bill charged me for period 6/01-21/01/22.
Phoned customer service and was told that any amount overcharged will be refunded.
Final bill in January credited amount?? (- £).
Information on my bill:
"Got a minus sign in front of your outstanding balance? That means you’re in credit and don’t have to make a payment"
I know I don't have to. I'm not your customer anymore.
When will I receive the money owed?

[#2]

Thanks a lot for getting in touch @Fil78

If the account still has an active Direct Debit Instruction on it, the monies will be refunded back to the account within 7-10 working days of the bill being produced.

If there aren't any Direct Debit details on the account, you should, in the next 7-10 working days, receive an email which will contain a URL, taking you to a webpage where you'd input your account details. If the email is not sent for whatever reason, you'll receive it via a cheque which will be sent to the billing address on the account.