Hi Rachel

I think you will be able to get more info on http://www.thassos.de . It's German run but you can translate the pages. There is a forum and the international section is in English and there's plenty of people who will be able to help you. There's also a webcam which I think is in Limenaria. We only stayed there for lunch so can't really tell you a lot about the place. The beach was quite good although pretty steep getting into the water.
I think going at the end of September you may be lucky and have good weather but on the other hand it could well be mixed. It will still be nice and warm I think though and the sea should be still very pleasant.

Hope this helps, Jean