Well you wouldn't know would you, it would sort of go against the whole ethos of espionage if the public knew everything. There are a number of unexplained deaths of Russians in the UK, who might or might not have been spies. We'll probably never know. Maybe you should apply your own 'return to first principles' and ask yourself why would a Russian mafia type go to such extraordinary lengths to use an expensive and deadly nerve agent, with all the expertise needed in procuring it, transporting it, and finally administering it, all at great risk to their own lives, when a simple bullet to the head in typical mafia stylee would suffice?...