I have recently had double glazing installed by a local tradesman. He did a very good job and all appeared well. However a few hours after he left a hairline crack appeared in one of the inside panes. My wife heard it go as she sat down to watch TV. I phoned the tradesman who came later that evening and he said it would not be a problem for him to replace. The next day he dropped off the invoice for the work. Am I obliged to pay in full or can I withhold some of the payment until he returns to replace the glass?