former first minister , who is bidding to become an mp , mocking labour after it slapped down its scottish leader jim murphy , who claimed he would write the party 's budget north of the border .
` i knew that -lsb- he would n't be writing it -rsb- already .
he also joked that he would ` check his top pocket ' -- a reference to a conservative election poster showing a miniature mr miliband in his breast pocket .
prompted david cameron and boris johnson to join forces to warn of a looming ` constitutional crisis ' .