Taxpayers who have not registered to file their 2007/08 Self Assessment tax return online can still do so but there is no guarantee that the activation code to allow filing on line will reach them on time. This is sent by first class post, but may not now reach the taxpayer or his/her tax agent before the 31 January deadline.
If a Self Assessment tax return is filed on paper after 31 October 2008 or online after 31 January 2009, a late filing penalty of up to £100 will be applied.
However if the tax owed for 2007/08 is paid by 31 January 2009, then the penalty will be reduced to 'nil'. The penalty is not reduced in the case of a partnership return. The latest update from HMRC on the Self Assessment deadline is available here