HMRC really doesn’t mess about:
If you miss the Self Assessment filing deadline you will be immediately liable for a £100 late filing penalty. The penalty will apply even if there is no liability, or if any tax due is paid in full by 31 January 2012.
These new penalties will apply to all Self Assessment tax returns from 2010-11 onwards. The fixed £100 penalty for failing to file a tax return on or before the filing date will therefore apply to:
• paper returns received on or after 1 November 2011.
• online returns received on or after 1 February 2012.
Daily penalties of £10 per day will also take effect if the tax return is still outstanding three months after the filing date. So if you file a paper return after 31 October 2011, you will be liable to a daily penalty on 1 February 2012 – that’s three months earlier than online filers. All the more reason to file your tax return online.
http://www.hmrc.gov.uk/sa/deadlines-penalties.htm#3
