Yes you need to fill in a Self Assessment Tax Form (or better still an accountant - he will charge - but this is also deductable) You will need your P60. Details of the mortage, the rent received, any expediture for the house, plumbing bills etc and then your accountant can calculate what you have overpaid.
I'd look at getting a buy to let mortgage - this may give you a little money at the end of the month for any unseen expense. At the moment you are at a loss every month and if say the boiler goes or something uninsured - you will have to fork out for it. Try and get a two year fixed rate mortgage which will give oyu something extra at the end of the month and one that you can change again in another two years say. Don't forget the mortgage rate has gone up three points in the last 6 months - if it goes up again you will be even more in the red each month.