The 2007/08 tax return must be submitted online to HMRC by the 31 January deadline. Filing a paper return will trigger a £100 penalty, as the new 31 October paper-filing deadline has now passed.
In order to file online, HMRC say that registration must be made by the Wednesday 21 January to allow enough time for the activation code to issue. Registration can be carried out here and click "Register for Self Assessment Online". Once this is completed, a pin number will issue in the post which may take up to seven days for this to arrive.
Full details are available here