With millions of people waiting to see if they are winners or losers in the fallout from the recent tax code realignment, Panorama reveals the situation inside HM Revenue and Customs. Insiders say the much-vaunted new computer system is making a bad situation worse, with billions in tax owed to the revenue having to be written off because of mistakes. Reporter Adam Shaw takes tax experts on the road to check people's tax codes, and hears first-hand from angry tax payers and from whistleblowers inside HMRC who complain that mismanagement, waste and staff shortages mean they're providing a poor service to taxpayers across the country.