If the first tax year that you let the property was 2003/04 (6 April 2003 to 5 April 2004) you should make a claim under the error and mistake provisions of the tax rules under Section 33 TMA 1970 as you normally only get one year from 31 January following the end of the tax year to make an amendment to a tax return. Section 33 TMA 1970 gives you longer.
I suggest you write a letter to your tax office setting out the revised figures. Make sure you keep a copy of the letter and a note of any telephone conversations you have with the tax office.
Tax Preparer
Consultant in Income Tax, Capital Gains Tax, Corporation Tax, Inheritance Tax