Make sure you email the person who is dealing with your recruitment on a regular basis, at least every 6weeks. I failed my first DMI, but if this happens, ask for feedback, they will happily give it to you. Take this on board and stay in touch with recruitment via email. This will ensure they remember your name, as there are hundreds of people in the queue.
From passing my assessments, took me 2 years to get in, would have been 6 months if I passed 1st interview.
Start researching the company NOW. The more you know the better. What trains they operate, which routes they run, (look at the Southeastern network map). What the role involves and why you are suited to it. Also learn the company values, they are really hot on this. They will give you a little info pack when you get an interview date.
