Thanks for all your suggestions..
I did the Lamma/Lantau/Cheng Chau runs, but with my isshy cantonese I could only find some pretty dull 6 person apartment things which were way more family than party
PDLM: My friend actually worked at that place for months, but didn't think about it because he just associated it with kids, but it was a throughtful suggestion..
rani: Concert looks great but is somewhat over our budget
masha: In the end we've booked out a stock 30 man junk... I'll send positive or negative feedback to the group so others can benefit.
At the end of my search I was pointed twice towards Sai Kung, I'll look into it for future events soon and pass on any findings..