Would be worth giving Adam or Kieran a ring at the Rib Centre as they sell them and see if a bung can be fitted to the hull. they recently moved to Eastlands Boat yard on the Hamble from Saxon Wharf so worth checking to see if you have the lastest phone numbers for them.
If you can't fit a bung low down, then I have a suction thingy for changing oil on an engine which works well at removing bilge water etc, just need a small hole in the deck above the lowest point in the hull.
http://www.nfauto.co.uk/oil_extractor.htm
Pete