Before 6 April 2008 HMRC normally had 12 months to enquire into a tax return from the normal filing deadline of 31st January following the tax year end.
This means if you are keen and filed your tax return early, say April 2007 HMRC could still enquire into your return 22 months later!
This caused many people to delay submitting their returns until nearer the 31st January deadline so HMRC had only 12 or so months to enquire.
From 6 April 2008 the 12 month tax return enquiry window starts from the date you submit your tax return and NOT from the filing deadline.
You can now file your tax return early and have certainty that 12 months later your tax return can be considered final and accepted.
It is true to say that HMRC are very busy as 31st January approaches so they are keen to find any way to encourage taxpayers to file their tax returns earlier.