Paul: If you are inclined to, I suggest you enter your measurements into Sailcut (http://sailcut.sourceforge.net/)
You will need the head length in addition as well as an opinion for leech round etc, but you will get an accurate surface calculation based on the foil shape. The link Wouter gave will probably give a somewhat different sum (but that's how they will measure the sail at regattas, so that is whats correct!). Playing around with how you think your sail looks in some saildesign software might be a real eye-opener with regards to shape and trim. Setting out to do a design for yourself, and comparing it to flying shapes you see on other boats will definatly be a learning experience (no need to build it). At least it opened my eyes for sail shapes and trim. Before I looked at sails and trim, but I did not really see what it was like. Now, after working some with Sailcut, I know more about what to look for and have a whole different 'eye' for sailshapes.

I entered your numbers into sailcut, and did some guesstimation on head-length and leech round. Came up with 12.93m2, but this is a number with seriously large margins for error as I did a fair amount of guessing without even trying to compare the outline to sails found in pictures etc!