DUNK1
From the rental income you receive for your house, you can set off the interest only portion of your mortgage plus any other allowable expenses such as repairs, maintenance costs, insurance, advertising etc.
If after deductions you are not making a net profit, then I cannot see how this can effect your tax credits. If on the form it asks for any other income and your net profit is £25 just enter £25, if net profit is Zero just enter Zero.
As veloxuk pointed out, do not forget to inform HMRC because it can have more serious consequences later on if you come to sell your home and you get landed with a large Capital Gains Tax bill, where this could have been reduced probably to zero by careful tax planning. If you left it till you actually sell the property and questions are asked about why rental income was not declared then it could cost a lot more to sort out with penalties, fines, interest etc.