A Phase III Randomised, Double blind, Placebo controlled, Parallel, Multicentre Study to Assess the Efficacy and Safety of continuing IRESSA 250 mg in addition to Chemotherapy versus Chemotherapy alone in Patients who have Epidermal Growth Factor Receptor (EGFR) Mutation Positive Locally advanced or Metastatic Non-Small Cell Lung Cancer (NSCLC) and have progressed on First Line IRESSA.