You have not stated exactly when you starting receiving rental income, but the normal rule is that you must inform HM Revenue & Customs (HMRC) by 5th October following the end of the tax year in which you started letting the property. The tax year starts on 6th April and ends on the following 5th April.
This rule would also apply if you started to receive any other untaxed income- such as you started your own business or you received untaxed income from investments. The same date also applies if you made a Capital Gain.
So if you started receiving the rental income before 5th April 2012, then you will need to register soon- by 5th October 2012. But if you started receiving the rental income after 6th April 2012, then you have until 5th October 2013 to register with HMRC.
Penalties can apply for late registration, so you should always register with HMRC as soon as possible.
Your local TaxAssist Accountant would be happy to assist you with your registration with HMRC and the preparation of your tax return.