Fret not! Have shown this to my husband for advice. He says no one pays for something they haven't seen/tried unless measurements/dimensions are a perfect fit especially as this is a large item. Do put a note through his door to ask if he would kindly bring the window round so it can be measured first before you 'enter into a contract' with him. Perfectly normal request. Keep us posted and know that you have donenothing wrong.
