Re-possession of flat from ex wife in Howrah/Kolkata

Both me and my wife were working professionals. Due to incompatibility, marriage was not functional. We got married in 1973. I bought a flat in 1990 in my name and we moved in (with our children). I took voluntary retirement in 1993. I moved out of the home due to constant fights the same year and started living at my sister's home with her family. She continued staying in the flat. We got divorced in 2002. She agreed to not claiming the flat as she was having an affair at the time (he lover left her later on). However, she continued staying in the flat. I have not received any rent/money from her yet. I did not ask her to move out because her income was low and she would have come under financial hardship at that time. But now, she has built a flat of her own and has retired from service last year. she is getting pension as well so she is not financially dependent on anyone now. Children are now grown up and settled in life. They have both moved overseas. I would like to re possess the flat so I can get either rental income or sale proceeds from it. Can you advise what will be the best way forward, how long it will take and an estimate of legal fees.