I've hung a big sheet over our kitchen windows, on the outside. The sun shines on that side in the morning. The sheet keeps far more heat out than the blinds on the inside do!
Yesterday, I did the same with the upstairs windows on that side. It makes a HUGE difference to the indoor temperature!
I would love to do the same to the front windows in the afternoon, when the sun shines on them, but it would be such an eyesore.
I'm still tempted though ...
In these temperatures, I leave upstairs windows wide open all night to cool the house as much as possible, but then as soon as it starts to warm up outside in the morning, I close things up. Curtains are closed for the rest of the day and the windows behind them only open an inch or so, to give some of the heat building up behind them a chance to escape.
I remember '76 well (I was 15), along with the ladybirds, and the Drought minister, Denis Howell, or H²Owell, as I remember seeing him referred to by one newspaper. 
Happy days! 