How can I repair my B&D drill?

I have a B&D model KR700 CRE 13 mm variable speed hammer drill which works very well except the Twistlock Plus keyless chuck now will not tighten or loosen on a drill. This is because the plastic chuck cover now does not grip the serrated ring mechanism inside the chuck when rotated. With the the plastic cover cut and removed, the chuck works OK if the internal serrated annular metal ring is rotated by hand. Can I purchase a new plastic chuck cover for this model or do I need a new chuck?
Any advice would be appreciated. Bill C