Thanks for the help Bregi, however I'm not quite sure I understand it to be frank (I'm still rather new to solving these sort of things, and probably haven't picked up on something obvious).
Looking at it logically, surely you must mean there is one number, not one letter... for example 1hundred... Or do you mean like 1l1 or something along those lines?
I have tried about 20 variations of the answer now, so surely there's something I'm not picking up on.