If a person has not notified HMRC by 5 October that they have untaxed income or additional tax due for 2010/11 they have a failure to notify offence. This would normally attract a penalty based on the tax outstanding on 31 January 2012, but this can be subject to a reduction for voluntary unprompted disclosure – which if done within 12 months of 5 October can be zero.
If a tax return was issued before 31 October then it must be filed by 31 January 2012 or a penalty is due. If no tax return was issued by that date then the due date is three months after issue and no penalty can apply until after that date – whether filed on paper or online.