I had the same problem. I read 3 different facts:
1) As France is an EU country as long as your passport is in date on your return journey then that is all you need.
2) A passport vaild for 3 months after your return date.
3) A passport vaild for 6 months after your return date.
I couldn't really get the "correct" answer from anyone and alot of people said it depends on which airline you are flying with (as we are going by air). So to be on the safe side I got out passports renewed.
I think you can apply for a new passport before your old one runs out and the Passport Office will put the remaining time on your new passport (up to 6 months). So, if your passport runs out in Jan 2009 and you apply now, they will put an extra 5 months on your new passport. I hope this makes sense, I'm not very good at explaining things!!