Are you sure the wrong position is on the plotter and not radar ? Is it possible the radar antena is misaligned in a radial way, I mean the error is included in a circle around the antena ?
No, my 2 compasses has a huge difference ... and can not understand why. If I change them from the left sideto the right and vis versa , the problem is still there .... The one from the autopilot is nearly correct