Please help! Property tax question......

Hi I am living in the uk but my parents lived in Poland - they bought a house in 2004. They both recently died and my sister and I went to the polish court and the house is now in our name. So far we have not paid any taxes such as inheritance tax it anything. We now have a buyer but the estate agent has said we will have to pay 19% tax as we have not had the property for 5 years even though mum and dad did. Is this 19% on the total sale price as thAt seems a lot!? Also the estate agent mentioned that if we use the sale money to pay off our mortgage then we don't pay the tax but how would that work? How would they know? Please help as I speak some polish but all the legal stuff is beyond me! Thanks