'fan or flue fault' iluminated,240v for a split second at fan then fault indictor iluminated. New pcb fitted and now hot water works fine but central heating fires up for a couple of seconds and then shuts down with flame failure indicator now iluminated as if the sensing electrode has'nt picked up the flame but hot water works fine.Thinking the new pcb may be faulty but any ideas on what may be wrong what i might be missing would be greatly appreciated