A Double-blind, Randomised, Cross-over, Multi-centre Study, to Evaluate Onset of Effect in the Morning in Patients With Severe Chronic Obstructive Pulmonary Disease (COPD) Treated With Symbicort®Turbuhaler® 320/9 μg, Compared With Seretide® Diskus® 50/500 μg, Both Given as One Inhalation Twice Daily for One Week Each.