See the Premier Guarantee advice on 30th May 2019 regarding use of Magnesium Oxide Boards https://www.premierguarantee.com/resource-hub/magnesium-oxide-board-technical-update/ 'In external framed wall constructions: The boards shall not be used as an external render board, or as a backing board for direct applied polymer, resin or masonry brick slips, unless they have specific approval for that purpose identified in the third party product approval certificate.'