Richardson is the Assistant Manager at Portsmouthm after ending a similar role at Chesterfield.
Richardson was previously the manager of Accrington Stanley having been appointed on November 1, 2012.
Leam first came to the Crown Ground as a player in 2005 and was part of the team that won promotion to the Football League that season.
He went on to make almost 150 appearances before taking temporary charge following the departure of John Coleman and Jimmy Bell to Rochdale in January 2012.
He later served as assistant manager to Paul Cook once again took caretaker charge after he joined Chesterfield.
His first game as permanent boss was at AFC Fylde in the FA Cup.