Run-in Period Following acceptable screening tests patients will complete a baseline run-in of 2 to 4 weeks duration prior to the study start. During this period patients will be required to keep a symptom diary and to record baseline lung function parameters using an electronic spirometer. As before, patients will be maintained on their normal inhaled steroids in doses equivalent to >/= 400 µg and </= 2000µg per day beclomethasone diproprionate and will be able to use short-acting beta2-agonists for symptom relief. No other asthma medications will be allowed during this period and the rest of the study. Use of all other, non-asthma, medication will be reviewed by the Investigator and allowed at his/her discretion provided a stable dose regimen is maintained where possible throughout the study.