Thanks for that! I managed to set an observer but I seemed to be getting an odd rotation and the camera wasn’t moving either.
I had a look in cardboard state and changed a couple of things - firstly I added a line to set the location of the observer to the world translation which put the camera in the right place.
Secondly I changed the order in which the quaternions are multiplied as the weird rotation was still there. I changed these around and this seemed to correct it. I’ve tested it both with an observer now and without and it seems to be ok. 
I’ve included the modified update method below if anyone else needs it:
// left eye
tempVars.quat1.set(context.getOrientation());
tempVars.quat1.toAngles(tempAngles);
tempAngles[0] = -tempAngles[0];
tempAngles[2] = -tempAngles[2];
tempVars.quat1.fromAngles(tempAngles);
tempVars.tempMat4.set(context.getLeftEye().getEyeView());
tempVars.tempMat4.toTranslationVector(tempVars.vect1);
if(observer != null){
tempVars.quat1.set(observer.getLocalRotation().multLocal(tempVars.quat1));
tempVars.vect1.set(observer.getWorldTranslation());
}
camLeft.setFrame(tempVars.vect1, tempVars.quat1);
// right eye
tempVars.tempMat4.set(context.getRightEye().getEyeView());
tempVars.tempMat4.toTranslationVector(tempVars.vect1);
if(observer != null){
tempVars.vect1.set(observer.getWorldTranslation());
}
camRight.setFrame(tempVars.vect1, tempVars.quat1);
tempVars.release();