Yes it will definitely be hot and its the busiest time. Its still the school holidays then, so like most holiday places it's peak season.
The season runs from beginning of May to end of October. In Marmaris, as it is a large resort there will still be bars/restaurants open outside this time.

thank you, I didn't want to go over and find places had started to close.

There will be plenty going on although by the first week of September Uk families with school age children will be going (you may find that a bonus). Same goes for Turkish families with children. The more mature visitors tend to still be around, younger ones with no kids and those with pre school children. The weather then is often fab with the extreme heat just toned down a notch. The sea will be very warm - lovely. Evenings do start to draw in a bit then though. However in my opinion it's one of the nicest times to go.
This year Ramazan starts on 1st September. It won't affect tourist resorts much but no doubt you know it is rather insensitive to walk around the streets eating, drinking or smoking. It won't affect the opening times of restaurants though they still go on serving through the day.

i was aware of ramadan. It has actually put us off going to Tunisia but I heard in Turkey it wont affect us as much. Of course we will not eat/drink etc in public. Is alcohol still available freely during ramadan???

Ramadan wont affect the tourist at all in the holiday resorts.
You will only be conciouse of it when a waiter tells you he hasnt had anything to eat or drink that day
and not all waiters adhere to it all the time.

Gene