Landlady not returning security money what can i do

I had rented a shop for retail business in October 2016. The agreement was done between me and the landlady for a period of 11 months and I had paid three months rent as security deposit. I had to close down my business after running it for 6 months, as it failed to take off. Now the landlady is not ready to return my deposit money despite several requests. The agreement clearly states that I had to give them a month's notice before vacating (which I did in written) and also that she will return the deposit amount after that. Her husband is responding on her behalf, to whom I have refused to talk to as the property & the agreement has the lady's name. kindly suggest the sections in which I can send them a legal notice. How it can be done ?