Some stuff from a site selling the polariser
These high quality glass filters manage the waveforms of light travelling to the camera from most reflective surfaces. Glare and reflected light can be altered by rotation of the polarising filter. The effect can be seen through the lens of reflex cameras as the filter is rotated. When used outside on sunny days, polarising filters have the effect of saturating colours and deepening blue skies. They can also be used in conjunction with other filters (although the polariser should always be placed in front of polyester filters to ensure it functions correctly).
Then I read this on the Lee website
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
Thanks for the reply :s
Facepalm moment