This study is designed to compare the effectiveness and safety of Fluticasone Furoate/Vilanterol Inhalation Powder (100mcg Fluticasone Furoate ((FF), GW685698)/25mcg Vilanterol ((VI), GW642444) or 200mcg Fluticasone Furoate ((FF), GW685698)/25mcg Vilanterol ((VI), GW642444) ) delivered once daily via a Novel Dry Powder Inhaler (NDPI) compared with the existing asthma maintenance therapy over twelve months in subjects diagnosed with asthma. This is a Phase III multi-centre, randomised open label study. Subjects who meet the eligibility criteria are randomised and will enter a 12 month treatment period.