Once you have received the CP - 88 Notice, you will have to gather past documents that are missing and file any past due tax returns. (taxdebthelp.com)
Obviously, the best solution for past due taxes is to pay the entire debt as quickly as possible. (irs.com)
HMRC say that between 1.7 and 3.5 million people will get a letter (technically known as a «P800») over the summer informing them they are due a tax refund for the 2010 - 11 tax year. (politics.co.uk)