Ian Blackford is stepping down from his role as SNP leader at Westminster, he has announced.In the run up to the next general election, that the SNP plans to run as a de facto referendum, Mr Blackford, who has spent five years in the role, said "right now is the time for fresh leadership".He said he would formally stand down at the party’s annual general meeting next week.There has been speculation in recent weeks that some SNP MPs were plotting to replace Mr Blackford as group leader.