We had the same problem with Lucy, and it was easily solved. We bought a short chain leash (small sized links), and attached this to the harness, then attached the normal leash to the end of the chain one. This meant that if Lucy chewed / tugged, it would be on a chain (rather than fabric or leather) - and it certainly worked to stop her! (As a bonus, the chain makes the whole leash longer and more manageable).
