If you make a mistake on your tax return, you've normally got 12 months from 31 January after the end of the tax year to correct it.
You may notify HMRC in the form of a letter to your Tax Office, detailing the omissions and which page numbers and boxes are affected. Or you may find it easier to write to them and enclose an amended return or just the particular pages that have changed.
Alternatively, if you filed your return online with HMRC, you may make any alterations online.
No penalties for late filing will be applicable, but please be aware that if the amendment results in additional tax to pay, you will be charged interest from the due date of payment. As 28 February has also now passed, you will be liable to a 5% surcharge on any unpaid tax. And a second 5% surcharge will apply from 31 July 2011 if it continues to remain outstanding at that point.
If you would like any assistance with this and liaising with HMRC, please feel free to contact your local TaxAssist Accountant.