What is the difference between Polar Pro ND filters and the Polar Pro ND/PL filters. The Vivid Cinema Series have ND PL written on them?

The ND/PL filters also include a Polariser in the same unit.       The straight ND is just that a neutral density filter only.

Ok cool thanks. I assume its like. Double lens filter. The circular polarisation turns around giving you different level of saturation/contrast?

I seen a you tube video showing that they dont have a marker as such to rotate from minimal effect to highest effect. You have to phyically play with it.

This might be a daft question? Does the PL unscrew off at all so to just leave the ND without the PL?  Or mayb it turns to a level where its like a ND without PL. When required rotate to get best from snow/water videos etc

No it doesn't screw off.
Yes you have to rotate it to get the best effect or effect you want.
They work best when the light source is at 90 degrees to the lens.
There is no markings. I've found the best way for me is to look through the filter at the subject I'm shooting and rotate it until I get the look I want then take note of its position and fit it to the Phantom .