HMRC has published an updated Internal Manual which provides guidance on the ADR mechanism. I have written about this in detail here.
What is ADR?
ADR is the involvement of a third party (a facilitator) to help resolve disputes between HMRC and taxpayers. It is mainly used by SMEs and individuals for VAT purposes, although it is not limited to these entities. Its aim is to reduce costs for both parties (the taxpayer and HMRC) when disputes occur and to reduce the number of cases that reach statutory review and/or Tribunal. The facilitator is impartial and independent and aims to assist both parties in resolving the tax dispute.
Source Marcus Ward
