Hello and welcome to the site. Thank you for your question.
You are correct in your understanding that you don't have to fill a tax return if you have income from UK property or land that is less than £2,500 after allowable expenses or £10,000 or more before allowable expenses.
Having said that, property income is taxable and you should have notified HMRC of this income. You say, for the first few years you purely covered your allowable costs and there is no profit. For the tax year 2013-14 (current reporting year) you have a small profit.
As you have never done a self assessment return, you should first register for self assessment on the grounds that you have property income. HMRC would issue you with a unique tax reference number (UTR) and it is a 10 digit number (e.g. 12345 67890).
As you would be completing a self assessment return for the first time, HMRC will normally send you a Tax Return for completion.
You can register for self assessment online and I am providing you with a link here
Now a bit of bad news, I'm afraid.
Late filing of self assessment tax returns carry a fixed penalty of £100.
More information on how penalties are calculated can be found herehttps://www.gov.uk/self-assessment-tax-returns/penalties
It is important that you get your tax affairs up to date.
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.