Forgot to includes mortgage interest expenses on self assessment form

Having submitted my self assessment form for the past 2 years which included income and expenses from property letting, it has just come to my notice that I have not included my mortgage interest payments, as expenses, and I need to rectify this. (Reason is that I was aware that a few years ago the rules changed such that landlords could no longer deduct mortgage costs from their rental income; however I had not realised that I was, and am, still eligible for basic rate tax relief on these payments.) Please could you advise how I should get these figures retrospectively altered on my last two self-assessment forms?

Hi Martin Pickles,
To make any changes to a tax return after the deadline please refer to our website:
Self Assessment tax returns
There is also information about tax relief for residential landlords and how it’s worked out at:
Tax relief for residential landlords: how it's worked out
Thank you.