In reply to ShropshireDad:
I've done last week of Aug/first week of September at the Tarn. I'm not a fan of hot temperatures but it was manageable. It will be hot but the guidebooks are pretty good at indicating the times each crag gets the sun/shade and all the crags are really close together so it's possible to chase the shade. We found that it's worth taking a very long lunch as even when crags got the shade in the afternoon they took a while to cool down from all the morning sun.
Take lots of chalk, be prepared to move around between crags, avoid slabs and enjoy a long lunchbreak (preferably with some paddling) and you'll have a great trip! Oh, there's some really cool caves nearby you can tour as well when the heat really gets to you and your tips need some time to grow back.