Thanks for that. I called a glass specialist who is going to come out and have a look. He suggested it could be a number of things. Including structural movement as already suggested, or a badly fitted screm under the edge of the panel. Also (encase it happens to someone else) he said the glass was toughened, similar to the side window of a car. If it were to shatter and fall on you, your injuries would be realativly minor compared to ordinary glass. Thank you for your help and advice, he's due next week. I will keep you informed of the outcome.
K.