Hi Mark,
The problem you described is easily solved with a gyroscopic sensor (or Gyro for short). Currently your radar overlay misalignment happens because your magnetic flux compass is not able to compensate for your boats pitch and yaw. A gyro unit does just that and allows for a significantly better synchronization between chart and radar on your E80. Raymarine sells a Gyroplus II unit (Raymarine product #E12101) that can easily be added to your existing S1 autopilot.