Also having large windows (which I have) increases interior heat by
solar gain, even if they are double glazed. When these flats were built in the 70s it was 'fashionable' to have extra wide windows - in the lounge and bedroom the windows occupy 90% of the horizontal wall space of the exterior wall.
http://www.greenspec.co.uk/windows.php
I have dropped and closed the venetian blinds to all my windows except the one I have to put the exhaust pipe through.