I was in the same boat as you last year with a D grade in Maths.
What i did was resit the exam when i went back to college. Dont worry you dont have to resit the whole year just do the exam again.
I resat once and got a C.
I think if you have your sights set on the BA sponsership then you need a c and above in maths so id resit the exam. Not only for the BA sponsership but it will help you in other things you do as it is a core subject therefore most places ask for it.
Good luck
Dave