Specially designed singing classes are clinically effective at treating mums with postnatal depression, a major three-year study has found. The authors say they could also be cost effective for the NHS at a time when mental health services are under pressure. At a children's centre in a housing estate in south London a group of 12 young mums sit in a circle on the floor as their babies cry, crawl and sleep on mats in front of them. But at this music class there are no bells or tambourines and