Single arm one stage Phase II study: post 4-6 cycles platinum doublet chemotherapy for patients with metastatic Non Small Cell Lung Cancer (NSCLC) offering Pembrolizumab as maintenance therapy to non-progressors with primary endpoint: Immune Related Progression Free Survival (irPFS) at 1 year. Aim to show that this is at least 25% (compared to an expected 12% 1 year PFS based on the Pemetrexed and Erlotinib maintenance trials).