Yep, I agree with hlb, no need to book and I doubt whether they'd do it anyway from my experience, though it's ages since I've cruised that area and things may have changed.
It was usually a bit of a friendly free for all towards the end of each day which is why we anchored!! Having said that, we did leave our boat in Vannes for a week back in '93 and it was very well looked after. Still didn't have to book though. Just drifted in, told them what we'd like and no probs.
We cruised this area about 6 yrs ago and managed to get in where we wanted most nights but it was crowded in August because thats French hols. Belle Ile was worst - a complete free for all to get into the locked basin. I would'nt have thought its even worthwile trying to book ahead as you could'nt guarantee to be in a particular place on a particular day as the weather is unlikely to play ball. Best you could do is ring ahead in the morning to find out whether the marina is likely to have spaces. Remember you'll probably have to speak French!
September should'nt be a problem