I once spent a week in a campsite near Caen in 40'C heat, at the start of July. People were getting carted off to hospital with heat stroke and we slept in our TT awning with two sides rolled up, it was far too hot to sleep in the cabin. It's not always cooler in Normandy!
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Funny enough, we also had a fortnight in hot weather one August at Chateau De Martragny, which is also near Caen. Decent enough site, good area - Caen is worth a visit, Bayeau (including the tapestry), lots of D-Day stuff and Mt St Michel is just 1.5 hrs away.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.