payment of stamp duty

My house was my PPR until I moved out and rented out my house. I now have to oay stamp duty on the difference between payment as A FTB when I bought it and payment if I was an Investor at the time I bought it.

Question is ....I should have done this last year so will I be charged interest and penalities and if so how much?

Also is there a form I need to fill in or do I just send the cheque to the Revenue?