A guide setting out those extra-statutory concessions in use at 6 April 2018 has been published by HMRC. The guide is available on the Gov.uk website.
An extra-statutory concession is a relaxation which provides taxpayers with a reduction in liability to which they would not be entitled under the strict letter of the law. Most are made to deal with minor or transitory anomalies under the legislation, or to meet hardship cases, where a statutory remedy is either difficult to devise or would run to a length out of proportion to the intrinsic importance of the matter. Many former concessions have been given statutory effect following the decision in the Wilkinson case in 2005.