The Inland Revenue has two deadlines for

tax filing.
If you got your tax return to them by 30th September 05, they will check and calculate your tax liability for you - this is a free service.
However, if you file your tax-return after 30th September 2005, you will have to calculate your tax liability by yourself.
The closing date for submitting all 2004/5 tax returns is 31st January 2006. If you miss the deadline you could be fined £100.