Today, we followed advice that we heard on the radio, which said:
as soon as you get up in the morning, open all your windows to let in the cool, morning air;
as the day warms up, close your windows to trap that cool air in your house/flat;
then, close your blinds/curtains to maintain that cool atmosphere.
And, do you know, it works! I have spent the day (it's now 1500 hrs.) indoors, in a pleasantly cool atmosphere, and my lovely wife has been outdoors, caring for her thirsty plants. She declared herself to be 'baking' when she came back indoors.