While finding out info for another thread I have found that the deadline for registering for self assessment for the 2012/13 tax return is today. This is particularly important for people who have received child benefit but now have to pay some or all of it back. www.hmrc.gov.uk/childbenefitcharge/declaringchildbenefit.htm
If you don't already fill in a tax return, you will need to register for Self Assessment.
You can do this by completing the online form SA1 Registering for Self Assessment (see the link below). HMRC will then set up tax records for you.
You need to register as soon as possible and no later than 5 October 2013 for the 2012-13 tax return. If you register late you may have to pay a penalty.
Once your tax records have been set up, you'll be sent a Unique Taxpayer Reference (UTR). You will then be able to sign up for HMRC Online Services and send your tax return online. There are lots of benefits to sending it online.
The deadline is 31 January 2014 for the 2012-13 tax return if you send it on online. If you send a paper return the deadline is 31 October 2013 - three months earlier.
If you want HMRC to include the High Income Child Benefit Charge in your tax code rather than paying it separately, you need to send in your return online by 30 December 2013.