Okay, this is a really peculiar problem I've got with one of the bikes.
Firstly it's not the freehub as I've tried this with two different wheels.
It's on a Ti456, when the rear QR is done-up (Shimano - Not very tight) the cassette/freehub will rotate only with a decent degree of force. With the QR released it's fine.
It's a Chris King ISO disk hub (both wheels i tried.)