From 6 April 2009 there is a change in the penalty you will pay if you are late notifying HMRC that you have commenced self-employment.
Up to 6 April 2009 the penalty was £100 and you had 3 months after commencement of trade to let HMRC know.
From 6 April 2009 the rules are changed as follows:
1. Anyone who ceases or becomes liable for Class 2 or Class 3 contributions must notify HMRC immediately.
2. A penalty may be levied (between 30% and 100% of the “lost contributions”) if notice is not given by 31 January following the end of the tax year in which you become liable.
3. There will be no penalty if you have a reasonable excuse for the late notification.