As per https://www.gov.uk/self-assessment-tax-returns/who-must-send-a-tax-return, if you get £2,500 or more in untaxed income "(for example from renting out a property)" you must complete a tax return.
I read that originally as rent, but as it goes on to say if the income is below £2,500 you should contact HMRC (who will presumably code it), it seems more likely it refers to profits (why not say profits...).
It then goes on to say you must complete a return if you have investment income over £10,000, which is again ambiguous (does that include rents?).
What I'm leading up to is, if you had rental income which exactly equalled your expenses (as maybe you just felt like letting out an unused property and you were a nice person who wasn't bothered over the money), would you need to complete a return? Would the position be the same if the figures were each £10,001 (or is income taken to mean profits again - I could have sworn a while ago there was a 'low rental profits are fine, but if rent exceeds £x you have to do a return regardless' rule)?