What is the maximum amount of time that a UK landlord can take to repay a security deposit on a flat?

I live in London and moved out of my flat almost a month ago. The landlord -- and the completely incompetent lettings agency called Inzo -- still have not returned my deposit. Furthermore, they are coming up with excuses to delay the return of the deposit. What is the maximum amount of time I have to wait before I seek a legal solution?|||I think you need attorney to explain the British laws which are different from American. I'm study words from British terms and very helpful!