Nihal Arthanayake gets rare behind the scenes access to one of the most secretive establishments in the UK - the Defence, Science and Technology Laboratory - which many of us know as Porton Down. It was here that the nerve agent that poisoned Russians Sergei and Yulia Skripal last year was identified as Novichok. He looks at the extraordinary range of things that happen at the DSTL - everything from police work to the armed forces, and new ways to prevent diseases that kill millions of people around the world.