Encroachment

Dear Sir, I purchased a piece of land from a society and later it got recognised by the local government. The issue is that when my house was built, the neighbour had already built his house. I lived in a different city and made only frequent visits. The construction was taken care by a known person. We moved into this property recently and still do not know who this neighbour owner is even now. He also lives in a different city. The issue is that, he has built the compound wall in my property. Not just that, the pillar which supports the portico rests on this compound wall partially ( approx 6-8 inches into my property). The portico overhang is also to the same extent. What do I do now. The property is about 15 years old. We came to know about this only recently. I have not asked any surveyor to measure it. I just took the reference of the house behind mine and indicating so. The tenant in that house is not willing to give the contact details of the owner. Kindly suggest. Regards SM