Suggest take an expert legal advice. Prior to going to a lawyer the following must be verified at your end.

1. Your title documents to the flat

2. Agreement with your tenant and the terms of the tenancy

3. Police FIR for tresspassing and illegal occupancy (your lawyer will guide you better)

These type of adverse possession cases usually take a lot of time to resolve and therefore be rest assured that the case might go on for a long time thus resulting in time and effort. In the event you have political connections try and discuss the matter with a local councillor who may be able to assist you, else try to resolve the matter amicably.