Quality of double glazed windows.

Not a DIY question but how does one know which double glazing companies windows are the best? For some reason the previous owner of our flat did not get the side window of the bedroom or the kitchen windows done along with the rest. Once when getting quotes for a previous house I went to a factory who made and fitted them. I was asked if I was going to stay long in that house. When I asked why he said that he could supply cheap ones that would look OK to buyer but not have the life of his better ones. Martin
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