This is pretty good Paul with excellent details identifiable when compared to the Hubble shot. It sounds to me like you have the basic technique down but by the look of those elongated stars you just perhaps need something that will align the frames slightly better..?
This was my effort a year ago using a normal CCD and stacking short single frame exposures (1s, 5s & 20s) with HaOIIISII filters at my native prime focus of 1120mm FL.
Mike