You can do your return using ROS. Yes it is free, but if you do not have an accountant you will need to register for ROS ASAP, in order to get yoyrself set up in time. I presume you have already submitted a TR1 form to your local revenue office, to register for self assessment.
While you do not NEED an accountant, you probably should get one, unless you are very familiar with expenses allowable against rental income.