how does it work over here, anyone know ? ie last year when I bought my new car it was an "F" plate, when does it change to a "G" ?? do they do it once a year ? is it the same time for cars and commercial vehicles ?

It is all said here.
http://en.wikipedia.org/wiki/License_plates_of_Spain
Or this one which seems to be more accurate and detailed.
http://www.geocities.com/amoros29/index.html

thanks so much, so from those sites it seems that there isnt a change at the start of the year, they just keep going til they get to the end of each letter/number combination

and saw here that the "G" registration started on 15th Feb so assume now all new vehicles will follow on

This site may also help:

http://www.aplicacionesinformaticas....dmatricula.htm

Mine was an F reg, and was Nov last year, think it's great that you don't have a change date, therfore there is no cachet in having the 'latest' letters etc, and, AFAIK, there are NO personalised plates here, and also no-one alters the spacing to make words.