I have used a Brompton saddle adapter pin http://www.foldabikes.com/CloseUp/bike/sapMain.html http://www.nycewheels.com/brompton-a...-adap-pin.html.

on top of a standard seat post , http://www.jensonusa.com/Seatposts/K...-Post-Seatpost

and a saddle clip, best one another Brompton product..

http://www.nycewheels.com/brompton-a...-adap-pin.html..

the clip rotated on top of the SAP, will allow the set back to be an adjustment,
combination of the clip location, on SAP, and the seat rails in the clip...