It is now mandatory that you submit your Supplementary Declaration by 21st of each month. This is one of the changes which has been made by HMRC, details of which you can find in Notice 60 Intrastat General Guide (April 2012). This is the definitive guide to Intrastat and it has been modified to cover a number of changes which are listed in paragraph 1.2 of the updated Notice 60.
If preparing Intrastat information is taking too much of your time, collating and consolidating information so that you can type it into the HMRC web site, it may help you to use GTA Online to manage this as a consequence of producing your export documents. You can check this out on the 4Xporters web site.