By Chris Dorrell Digital advertising group Brave Bison has abandoned its attempts to take over Mission Group, criticising the firm for refusing to engage in “constructive dialogue”. The AIM-listed firm submitted two bids for Mission Group, with the second bid representing a 55 per cent premium on Mission’s pre-offer share price. Brave Bison also intended to pay down a portion of Mission’s £26.8m debt to create a “sustainable capital structure”. Last week, Mission Group, a digital marketing firm, said the offer was “opportunistic and significantly undervalues the group“. It argued that Mission …