Did you ever solve the problem of turning off the amber warning light / parkbrake failure message? My 2008 XJ8 Vanden Plas has the same problem. The code is c1799 (hall effect failure). Crazy thing is my EPB works just fine, both automatically, and manually. The hall effect sensor is inside the EPB actuator (motor) and I really don't want to drop the subframe and gas tank just to replace a motor that is working fine, simply because the hall effect sensor inside the motor is not working correctly. I just passed my Nova Scotia motor vehicle inspection, because, as the mechanic said "the Electronic Park Brake works just fine - you simply have a stupid light on your dash that won't go away"!