https://www.gov.uk/government/publications/tax-avoidance-tell-hmrc-about-an-indirect-tax-scheme-you-promote-dasvoit1
Tax avoidance: tell HMRC about an indirect tax scheme you promote (DASVOIT1) - GOV.UK
Use form DASVOIT1 if you're a promoter and need to tell HMRC about a VAT, or other indirect tax avoidance scheme under the disclosure of tax avoidance rules.