I solved all the clues and expressed each of the two-letter ciphers in terms of one-letter ciphers. I was then able to fill the grid with the single letter ciphers and it was relatively easy then to deduce the code. The only thing I dont understand is the title, which I have enciphered. The first five letters make sense but not the final three.
An ingenious puzzle which looked impossible to begin with. Fortunately most of the clues were easy, which helped enormously.