To clarify Kingmaker's comment about not needing to complete a tax return if no request has been issued. You should tell HMRC that you are receiving income from land or property using Form SA1. HMRC can then decide whether or not they want you to complete a tax return. Do not just assume that they know - because they don't.
By completing a tax return in respect of property income, even if you don't have to pay tax, you are reducing the risk of having your time wasted by an unnecessary enquiry. HMRC check Land Registry records and local authority information to see if ownership is different to the occupier and where they can't see a tax return from the registered owner declaring property income, they can target the owner for an enquiry.
HMRC also receive information about landlords from letting agents so, again, if an agent tells them that you are receiving rents but don't declare those rents on a self assessment return you are likely to be targeted.
It may be that you genuinely don't owe any tax and the enquiry will conclude that, but you will have spent an awful lot of time getting to that stage. HMRC are not very understanding people and won't just accept your word that you haven't made sufficient profit to pay tax; you will have to prove it.
And we find that a significant number of landlords who aren't making a profit are actually making a loss. Any loss can be carried forward to future years to reduce tax when they eventually make a profit. If the loss hasn't been recorded and carried forward it can be very difficult to get the relief - mainly because you won't know what it is.