It's the perfect time of year for a celebratory gin-and-tonic, and these days the tipple raised at Christmas may well have been made in London. After centuries of decline, the British capital's distilling industry is picking up again, fuelled by small-scale producers and European rules changes that recognise 'London dry gin' as a distinct category - along with a worldwide uptick in the demand for premium spirits. Peter Day meets some of the entrepreneurs behind the trend.