Our holiday home, a flat in Spain is closed up for up to 6 months during the winter. When we open up the bathroom and kitchen are full of tiny black flies. It would appear they are coming from the drains, as there is a concentration in the toilet area (plugs are left in the bath etc) and around the kitchen sink. What cn anyone suggest to stop them getting in. Most of them are dead by thte time we gett there, so it's prevention, not cure that's needed! We have also found that after only a week or two closed up during spring/summer, threse little devils reappear. All suggestions gratefully received.