For nearly 40 years, East Germany's Stasi was one of the most formidable secret police forces in the world, with a vast network of spies and informers reaching across the globe until the fall of the Berlin Wall. As the Cold War ended, scores of them were still operating in the UK. With access to recently opened court files, Julie Etchingham reveals some of the Stasi's UK operations and asks why the identities of many alleged British informers are still a guarded secret 30 years after Germany made its file public.