Although i haven't come across an adjustment for french doors (mine just have hinges no adjuster) I rang Anglian windows to ask their advice.
The technical department don't work weekends so couldn't get you a definitive answer, but the phone operator seemed convinced french doors can be adjusted.
Ring your supplier, or pick one out of the phone book and pretend they supplied you, and they will talk you through the process.
If a wooden door had dropped I would adjust the hinges by either packing out the bottom one so that it stuck out more which in turn pulls the top of the door in. Or remove the door and sink the hige at the top into the frame or door more, pulling in the top and pushing out the bottom.
As your doors are UPVC you won't be able to take a wood chisel to it to remove wood, but you could take the offending door off and remove the hinge, find some plastic about the same size and place the hinge on top.
Now draw around the hinge and mark the screw holes, then trim the plastic to size and drill out the screw holes. replace the hinge on the frame (not door) carefully putting the packing between the hinge and door frame first.
You should now be left with the door with one hinge on, one off, with help offer the door up to the hinges and relace the screws.
Naturally the amount of packing needed depends on how far the door has dropped so you may need several pieces! I'd make the packing out of any old sheet plastic such as old tupperware etc.