On a German weather site:
http://www.wetteronline.de/cgi-bin/regframe?3&
PRG=klimavar&LANG=de&WM... &ART=WST&ZEITRAUM=04&MONAT=09&JAHR=2001 you can see how windy it was in september other years.
sep 2001: lot of 3-5 sep 2000: first 2 weeks easy, last 2 weeks windy sep 1999: lot of 3-4, some days with more, some with less wind.
In La Manga you never know, in september usually wind is between force 3-5, but you can have 2 weeks with onlyu force 2-3, or several days with force 6-7
Check
www.club-vela.com for a nice surfing place in La Manga! I have been working (and windsurfing) there for several years.
Just Amsterdam