18c over night is high, very high to be honest, ever thought of individual rad stats, they may help keep the rooms you want higher but keep the bedrooms cooler, no point in heating a bedroom in my opinion, your warm enough as it is! Have to be honest your costs are higher than even what i pay in the summer months for the aircon, its normally no more than a tenner a day. It does sound like your house is very poorly insulated.