Lee Filter Vignetting

I recently bought a Lee filter system with grads and the universal hood and polariser. While the Hood and grads work well with the Zeiss, unfortunately the 105mm polariser, that screws into the front of the hood, causes hard vignetting – it is not just a gentle darkening. See picture. I am investigating a slimmer filter to see if this will solve the problem, with Heliopan being top of the list at the moment.