By Charlie Conchie Sir Graham Brady and some of Britain’s biggest firms, including Aviva and BAE Systems, are piling pressure on the chancellor to overhaul so-called share incentive plans and unlock a “new wave of employee share ownership”, City A.M. can reveal. In two separate letters to Jeremy Hunt, seen by City A.M., a group of Tory grandees and some of Britain’s biggest listed companies have called on the chancellor to slash a five year ‘holding period’ in the schemes that they claim is hampering investment. Share incentive plans offer workers a stake in their employer and allow them to re…