Hi Rob,
I have not gotten into this field yet but may i suggest taking 10 shots at 4mins. and stacking rather than 1 long exposure. I have watched these type of threads for a long time looking for the HOLY GRAIL!!
But this seems to be the way.
From someone that DOES NOT KNOW THE WAY,LOL.
I've also had a few, QUITE A FEW,LOL,also i agree at 800 ISO
Cheers Duncan