As millions of Britons receive letters telling them they've paid either too much or too little tax in recent years, Simon Cox investigates what's gone wrong at Her Majesty's Revenue and Customs, and reveals how the back log means there is a tax write-off still to come.

According to the Government, a new computer system has, for the first time, been able to reconcile the ordinary citizen's tax affairs and give an accurate estimate of what he or she owes the state. But is that the whole story?

With the testimony of insiders, The Report paints a picture of chaos, staff shortages and crisis management. It examines how delays in the complex IT project led to almost four billion pounds of miscalculations on tax owed to the Exchequer. And Simon Cox reveals that amongst a backlog of 15 million open cases from over 2 years ago, 1.5 billion pounds of underpayment by taxpayers is about to be written off because of fears of legal action.

Producer: Rob Cave.