By Lars Mucklejohn Mission Group has rebuffed a takeover proposal from rival AIM resident and digital advertising company Brave Bison, which acquired Steven Bartlett-founded Social Chain last year. Mission announced on Monday that it had received an unsolicited proposal from Brave Bison on 29 April. The all-share proposal had an exchange ratio of 11.5 Brave Bison shares for each Mission share, which valued the latter at around 29p per share. The firm said that after taking financial and legal advice, Mission’s board unanimously rejected the proposal. It said it considered it: “opportunistic an…