Two years ago, Russian opposition leader Alexei Navalny died in a Siberian penal colony. There was an outcry and many suspected foul play, but nothing could be proved. That is until last weekend, when five European countries including the UK announced that they had made a startling discovery: Navalny had been killed with a rare frog poison. How was the poison was identified, how were the samples smuggled out of Russia, and why does the evidence point directly to Moscow? Venetia and Arthur speak