right chaps, have a problem an im not 100% what to do

driving the car yesterday, and i notice the water temp slowly start rising, and then the water leve indicator started flashing, then worst happend and loads of steam poured out of my bonnet, but from the expansion tank area.
so i pulled over an popped the bonnet.
looks like the water is escaping from the expension tank. so i dropped the water out and refilled, ran the car upto temp with the expension tank cap off and topped up when nessesary. but as soon as it got to temp it just boiled over, and the fan didnt even have chance to kick in... ive just realise that i didnt have the heaters on so could that have caused it to boil over, and it simply bea case of the seal in the expansion tank cap is perished?
cheers
dabill