my DD was born in the hottest summer on record: 2003. i found a towelling sleeping bag in John Lewis that was perfect for those warm summer nights. we'd just put her in a short sleeved baby gro and light trousers, the towelling sleeping bag, and used towelling sheets in her cot (mothercare sells them). crack the windows for airflow and she dreamed away.
try grobag.com, they have 0.5 togs for really hot nights. theres lots of info there about how to dress baby when its hot, they provide little thermometers for the room that indicate what nightclothes to put baby in and with what tog of bag
I have also a towelling grobag (1 tog) but very fresh in summer.
Somebody mentioned here about a good way to keep the rooms cool: Open the windows just early in the morning (and all of them at the same time so the air really cool the house), as soon as the sun starts to enter through the window, close the windows and curtains/lower the blinds. DS has a southfacing room that gets the sun most of the day and still is very cool with this method. I think that a blackout rollerblind (aroun 10 pounds in IKEA) does the work beautifully.