I'm 100% in agreement with "BOAC" on this one. The reason for having this clause in the regulations is simply to put into place a structure to prevent companies from forcing fatigued or unfit crews to fly or work unreasonably long hours. It is also there to permit duties and rotations to be completed, following unforeseen delays or extensions. The spirit of the rule needs to be applied and anyone using it to rush home to make the party in time is simply abusing the system. In my airline, anyone refusing to extend under unforeseeable circumstances is given the opportunity to consult the company doctor. Most of our crews do appear to interpret the rule this way however, I have noticed on occasion that our British crews do appear to do a lot more complaining with regards duty extension than anyone else.