On 5 September 1785, a trial began in Paris that would divide the country, captivate Europe and send the French monarchy tumbling down the slope towards the Revolution. Cardinal Louis de Rohan, scion of one of the most ancient and distinguished families in France, stood accused of forging Marie Antoinette’s signature to fraudulently obtain the most expensive piece of jewellery in Europe – a 2,400-carat necklace worth 1.6 million francs…