Landord wrongly deducting deposit

My current house lease contract is from 1 July 2014 to 30th June 2015 with a lock in period of 6 months and notice of 2 months. So by the terms of the agreement my lock in expires 31st December 2014. I gave my landlord a two month notice on 12th December 2014 by email, which means i can vacate on 12th February 2015. The landlord, now realising that he will not be able to put the house on rent, said to me that you cannot give me notice during the lock in period, even though 2 month notice expires much after the lock in period. In saying so he asked me to come and give him a physical notice on 1st Jan, which i did but on that same day he said i am ok if you wrote that the notice was given by email on 12th December. Now i have a signed letter from him saying please accept this letter as 2 month notice starting from 12th December 2014 sent by email and he signed this letter on 1st Jan 2015. Now that 12th Feb is close he is backing off from the letter and is saying that you will have to pay full months rent or else he will deduct from my deposit. What can i do?