Andy,
I have a bit of experience with magnetic compasses from my time as navigator on the aircraft carrier USS NIMITZ. Historical photos show a binnacle type compass back in the 70's when NIMITZ was first commissioned. However, during my time onboard (2005-2007) we had an "electronic" magnetc compass that needed to be as far from any magnetic disturbance, as high up and as close to centerline of the ship as possible. That's pretty tough for an aircraft carrier since airplanes land on the centerline. Ours was installed in a remote spot on top the island (superstucture above the flight deck) as close to centerline as we could but we had to calibrate it all the time. I would think that with the majority of QE2's superstructure being made of aluminum that they had pretty accurate magnetic compass readings.
Thanks!