The 31st January is the deadline for submission of your self assessment tax returns if filing online.
If you have not filed online before, are you aware that you need an activation code supplied to you by the HMRC which is sent via snail mail and can take 7 days+ to arrive.
If you have not applied for one of these activation codes then I suggest you get your arse in gear and do it now by following this link. Anyone that files their returns late will receive an automatic £100 penalty!
The 31st of January is also the deadline for payments due for tax year 08/09 if filing online
If you miss this payment date interest will be added to the amounts outstanding!