I grew up in the south of Spain, where it easily reach more than 40 degrees. Usually "worst" month is July because August usually is "colder" during the night. The feeling can also vary depending of the place you are, for example, where I grew up, Zafra, a small city in the south of Extremadura the weather it's very very hot, but it's dry, so it's much better than, for example, Badajoz, very near so similar temperatures, but terribly humid in summer.... horrible.
If you want to sleep confi in your Cali be sure it's most of the time in the shade and, if possible, when the sun is gone, refresh your Cali with water using a hose.
Try to leave opened as many windows/doors as you can (it will depends of how safe the place is) and use some fans. There are also a metallic device that allow you keep the tailgate "open" but secured.
I understand for you Britons 40 degrees is too much, and maybe you should use the air con time to time. I prefer not to use it, don't like too much, but sometimes is unavoidable.