Good work Clive. Tough target at the best of times. Is there a reason why you didn't raise the black point?
EDIT: I think this is the first time I've seen someone post details of their calibration frames. Something I just take for granted... considering its the light frames that count. LOL.
Anyway, you may wish to check this flats analysis on the ADU, quantity to make a master -
http://www.darkhorseobservatory.org/dark_horse_observatory_22oct07_021. htm
Point being, don't waste too much time going for "bucket loads" of calibration subs when the value returned is minimal.