[chromedog]

Hello there, just a quick clear up of rules please, am i right in understanding tht you can embark or disembark from a vehcal no matter how far it has moved.

Not quite. If it moves up to 12", you can disembark.
You can also NOT embark and disembark in the same turn with the same unit.

Eg. My banshee moved 18inches in a WS and then wanted to disembark, would this be aloud and then would another unit then be eligible to embark from a WS that has just moved 18inches.

You can't embark or disembark into/out of a vehicle that will move at flat out that turn (over 12"). So that's a no.