In reply to Pete_Robinson: Video link which might answer some of your questions
http://www.leefilters.com/index.php/camera/polariser#nd-videobox
I have a lee 100mm filter system I mostly use a big stopper and resin 0.6 and 0.9 soft ND grads very rarley use a polariser though you can buy one for the lee system which screws on the front of the adapter (see the Video)
I have used a Hoya Polariser with the lee adapter usually taking the lens UV filter off first to cut down vignetiing, then screwing the lee adapter holder onto the polariser, you have to be carefull your not rotating the polariser when your rotating the clip on filter holder.
The Lee system is a bit dear but it is good.
Combining with Polyester Filters
If using a polarising filter in conjunction with polyester filters, the Polariser must be placed in front of the polyester filters, not behind, otherwise it will not function correctly. This problem does not arise when the polarising filter is used in conjunction with resin filters.