For the next two weeks of the study, subjects will be randomly allocated to take either their regularly prescribed inhaled corticosteroid (ICS) and a long acting beta agonist (LABA) or their regularly prescribed ICS and tiotropium. At the end of this period all subjects will return for spirometry, urine sampling and for measurement eNO. This will be followed by a two week wash-out period when all subjects will take only their inhaled corticosteroid (ICS). For the subsequent two weeks the subject will take the treatment they did not receive in the first two weeks. At the end of this period all subjects will again return for spirometry, urine sampling and for measurement of eNO. During a fifth period of two weeks duration, subjects will take both the LABA and tiotropium. Subjects will then return for final measurement of spirometry, urine sampling and eNO.