Stephen Sackur speaks to a novelist whose fictional world is filled with drugs, sex, sleaze and alienation. Scottish writer Irvine Welsh draws deeply flawed characters and makes them entertaining and all too human. His first bestseller was Trainspotting, a tale of heroin-ravaged youths from the wrong side of Edinburgh's tracks. His latest book returns to the same turf. He now lives most of his life in the US, so why is his imagination is still so heavily stirred by Scotland and his working class roots?