My German colleagues tell me quite clearly that you only get ONE shot at the DLR test, no matter which airlines you may be applying to at the time or at a later date.
Thus I fear it is too late for you but the advice they have for others is to make sure you are properly prepared for it in advance, and if necessary do not be shy about delaying the test date if preparations are not complete as your chance with many of their national airlines will depend on it.
Sorry it's bad news, hopefully others may benefit from your experience.
SJ