Glass roof panel

[Fred]

If exact size isn't important why not contact a double glazing company in your area that might have the odd rejected units ( when I say rejected I don't mean faulty ) but made to the wrong size ....they do get them occaisionally and worth an ask

That could be worth a try, but we don't have many glazing companies around here and knowing the French it would be double the price of a made to measure panel.  

The French seem to have this thing that second hand/reject/etc is worth a Kings ransom. For example a car you would pay to have taken away in the UK would be advertised at €10,000000000 here, and they would sit on it for 100 years till it sold.