LM Temps in September should be about 1 degree cooler, certainly when we passed through last September they didn't seem a lot different to the average June temp, leaving aside any extremes. the only difference will be the hours of darkness, so it will get cooler earlier and warm up a bit later.
If the Spa 24 goes ahead at the end of October it'll be interesting to see how they (and the timing) handle the clocks changing!