My drivers side is sitting one and a half inches lower than the passenger side both front and back (measured from ground to bottom of wheel arch). I've got ACE but no air suspension. How do I tell if it's the ACE, Dampers, or Springs without dismantling too much - and whats the chances of front and back failing together ?

It is nothing to do with ACE, its more likely a broken road spring - a careful examination of the springs on the low side should reveal the culprit.ACE is only active when the car is moving.