Here at the Beeb we've spoken to an XL Germany spokesman who has given us the following:
The plane was being returned to an aircraft leasing company. It was being given a maintenance check at Perpignan airport.
The aircraft appears to have crashed into the sea off the south east French coast during test flights following the maintenance work.
XL Germany says there were two flight crew from XL Germany and 3-5 engineers on board
Hope this helps.