From that date, returns lodged by the normal due date will only be ‘open’ for an HMRC enquiry (assuming there is no serious tax irregularity) for one year after they are filed, as opposed to the previous system under which they remained open for a year after the due date for filing.
For example, a corporate tax return filed on 1 May 2008 that is due to be filed on 1 July 2008 will only be open for enquiry until 30 April 2009. Previously, such a return would have been open for enquiry until 30 June 2009.
The position for returns filed late is that they remain open for enquiry until the end of the quarter (the quarter dates are 31 January, 30 April, 31 July and 31 October) following the anniversary of the day on which they are filed – i.e. a return due on 1 October 2008 which is filed on 1 November 2008 will be open for enquiry until 31 January 2010.
HMRC Corporation Tax Return Forms