Getting Refund From British Gas for Over-payment?

I tried posting this in UK Personal Finance, but it was blocked by mods. Posting here even though the other sub seemed better suited............... Apologies if this is the wrong place. My mother moved in to her flat in late January 2021. She has always received her combined bill from British Gas addressed to 'The Occupier'. She tried to change this to her actual name 2 years ago, but to be honest, gave up dealing with their customer service etc. So, the bills/statements have been coming quarterly to 'The Occupier' since February 2021, she has gone to the Halifax and paid them. Today she has received a statement with the information she has overpaid, account is in credit, by £1511.31. Will it be possible to get this refunded to the method of payment used the last 2 years? She will be vacating the property within the next 1-2 months, so, just leaving the £1511.31 on as usable credit, isn't really what she wants....... I used British Gas' online chat tool, and was told she needs to make a new account, as if she just moved in, and provide metre readings. But it doesn't look like I can input the move in date as January 2021. The person on the chat essentially said, make a new account, the old account will be merged, therefore the credit will be refunded to the new account, in her actual name. It's a significant amount of money, so we want to know for she actually gets it refunded. Anyone have any experience similar? Will making a new account 'wipe out' the old, 'The Occupier' one, with the credit? Thanks!!