http://www.cesifo-group.de/ifoHome/publications/working-papers/CESifoWP/CESifoWPdetails?wp_num=6438&CESifoWP.search=+
https://www.nao.org.uk/wp-content/uploads/2017/07/The-Customs-Declaration-Service.pdf
https://www.gov.uk/government/news/david-davis-opening-remarks-at-the-start-of-the-third-round-of-eu-exit-negotiations