Flat let out without rental agreement

I & my wife own a fully furnished residential flat in a co-operative society in Mumbai. My flat purchase agreement is properly registered and the electricity connection from of Reliance is also in our joint name. We gave it to a couple on rent 28 months ago. We personally knew that couple and they seemed to be trustworthy. So we decided to go ahead without having any written rent agreement. They also were ready to pay the rent in cash. So at that stage we thought it is good us – no hassle of agreement registration as well as receiving rent in cash was a good thing for us. The only thing through bank transaction is the deposit money Rs. 90,000 that they have paid vide cheque favouring me given 28 months ago. In short the let out was without anything in writing and based on mutual understanding that from either side we can inform other party to terminate this arrangement (i.e. letting out) with 2 months notice. They have been very regular in paying monthly rent in cash – for which also they never asked for any cash receipt from us anytime in the past. About 4 months ago, we orally informed them that we now need to terminate this letting out & that within 2 months they need to vacate. They said ok but towards closure of 2 months said they couldn’t find any other suitable accommodation and hence requested to extend their stay for some more time. They did that for 2 more months and have been paying rent on time as earlier but are very very reluctant to move out. I’m now afraid that they just don’t want to vacate and want to continue for much longer period with or without paying rent. Please let me know how can I force them to vacate. Thanks in advance.