Andy Warhol’s Shot Sage Blue Marilyn fetched £158 million in May at Christie’s New York to become the most expensive 20th century artwork sold at auction, overtaking Jean Michel Basquiat’s Untitled which sold for £90 million in 2017. In fact Warhol and Basquiat knew each other and they even worked together. The artists were introduced by art dealer Bruno Bischofberger who persuaded them…