Unable to Access Business Tax Account for VAT

Hi, My accountant has registered the business for VAT recently. We have now received the VAT number and certificate. I have tried to add VAT to my business tax account, however it tells me "Your business is set up for VAT with another Government Gateway user ID. Try signing in again using a different user ID." The problem is I have never added VAT to a business tax account and neither has my accountant. I have called the VAT online services helpline and failed security as we didn't know the box 5 figure. This is strange as we have never submitted a VAT return? We tried £0 but it didn't pass. I also received a letter from HMRC recently with a cheque for a repayment / advance of statutory payment of £9,000, the letter includes the VAT number. This is also strange as we have never submitted a VAT return so why is there a repayment? Do you think someone could have linked my VAT number to their own business tax account and is now using it to reclaim VAT fraudulantly? What should I now do?

Hi
Please contact our VAT team so that we can investigate this.
VAT: general enquiries
Thank you.