Olly64, it is unusuall for them, Most windscreens are bonded in to cars and you use a special sealer to do this. Sometimes what happens is the sealer gets into a run off chanel and blocks it up and the water is forced to go inside the car, Sometimes its the windscreen companys fault sometimes people don't listen to what there told ie. avoid putting on your wipers, washing your car, even getting your car wet, don't hit pot holes etc for 24hrs after the replacement has been fitted. It's sometimes to hard to do all these but most people get away with it. The company might have a mobile unit where they can come to your house or job and fit the screen with out you taking a day off.