A tax enquiry is the process by which HMRC check in detail that the information on a tax return is correct and complete. They do this by asking questions about your return, through meetings and a review of your records. HM Revenue and Customs has the right to open an enquiry into any return.
Because any return can be selected for an enquiry, you need to keep all the documents and information you used to prepare your tax return [until the time allowed to HMRC to make an enquiry is past]. HMRC can seek a maximum penalty of £3,000 for not keeping these records, though this is not applied automatically.