The business secretary has been forced to admit the existence of a previously secret package of state aid to Nissan that could have been worth up to £80m had the carmaker gone ahead with plans to manufacture a new model X-Trail in Sunderland after Brexit. Greg Clark released a letter dated October 2016 in which he pledged tens of millions of taxpayer support and promised the Japanese company it would not be “adversely affected” after the UK left the EU.…