Aural is not only the singing bit !!! There's rhythms and questions that you answer too, so really, the sight singing and singing back a phrase is only a few marks.
Try and but the aural tapes for the grades you work on. They are really good, and are soooooooo helpful - I've used them for every exam i've done !! Using them, practice singing, untill you can do it !!
Good luck !
love piano_ellie xxx