Plessey Co Plc has filled the post of managing director left vacant by the resignation of Sir James Blyth a year ago by promoting finance director Stephen Walls: at 41, Walls, who was recruited from Cheeseborough Ponds a year ago last July, and says that the company still wants to expand its computer services arm after the acquisition of Hoskyns Group Plc, is clearly a candidate for eventual sucession to chairman supremo Sir John Clark.