By Ali Lyon Leading City figures have sounded the alarm over the health of London’s AIM market after a new analysis revealed the number of companies quoted on London’s junior market to be at a 20-year low. In another sign of the malaise afflicting the UK’s capital markets, accountancy group UHY Hacker Young found that roughly 80 firms have left the AIM exchange in the last year. The exodus means that just 722 PLCs remain on the bourse; the fewest since 2002, and less than half the number that existed during the market’s heyday in 2007 when 1,694 companies were listed on the growth-stage market…