Hi Emma12,
If you have been accounting for VAT incorrectly as a UK established business then you will need to correct this as an error correction.
Please see the guidance below:
4. Correcting VAT errors on a return already submitted
You can only do a correction for the VAT which you know you have overdeclared by as this would be your responsibility, not the responsibility of the marketplace.
It will be the responsibility of the marketplace to account for the VAT correctly from 2020 and going forward.
Please make sure that we have the correct address details for you and that we have the correct status for you as a NETP.
If there is any doubt with your status please contact as at:
isbcvatestablishment@hmrc.gov.uk
Thank you.