In this region there are two different billing systems for digi & telco so that is why you need two dd's each month, the digi one can be completed over the phone whereas the telco/bb one needs to be recieved on a mandate before it can be set up, which is why it can be delayed sometimes.
Have you ever recieved anything at all at your address from ntl, if not then it seems that there is clearly some glitch as each system generates bills seperately, so it may be worth checking how your address is showing on your accounts.
edit: just re-read your post & see that you've already checked the address
, very strange, don't live in a flat do you?