In this study, the investigators will aim to identify children with asthma whose symptoms are poorly controlled on inhaled long-acting beta2 agonists. Via a simple saliva test, the investigators will identify the presence or absence of the specific genetic change potentally influencing the effectiveness of treatment with salmeterol or related longacting beta2 agonists thus enabling the investigators to recommend either salmeterol or an alternative medication for the treatment plan such as montelukast. The investigators will randomise the patients into two groups; to receive "personalised care" where the choice of controller medication would be based on the child's gene test results and predicted response to long-acting beta2 agonists, or "standard care" following the BTS guidelines at the clinician's discretion without knowledge of the gene test results. The investigators aim to measure whether this individualized approach to asthma prescribing results in improved control of asthma symptoms and overall quality of life. Targeting treatment to a child's specific genetic make-up is a concept known as "personalised medicine".