Once they are off open the rear door or lower the window and slide the bow out of the curtain. At this point you want to have some kind of clip ready to secure the curtain and prevent it from rolling inside the roof at which point your job would get much more difficult. I used a clip we use in kitchen to close open flour or cereals so they don't get damp. Once put together the bow looked like this. (I forgot to take pictures of bow with the aluminium bar, but you get the idea.)

I'm hoping your fix is still working. Your solution uses an interesting piece of metal... but I'm wondering if it would hold up OK, as the metal would be exposed to the sun and heat up. That heat could cause the glue holding it to the plastic to weaken and then the metal would detach at some point. Hopefully that didn't happen and that your repair is still holding, nearly 3 years later. It is necessary to say that this repair is not applicable for everyone as the way the latch snaps is different every single time. What I would like to present is a solution I did on my car. You can get the idea how to proceed and see that it is not that difficult in the first place.

At this point you are going to improvise as I said at the beginning every snap is different. For inspiration below is a solution I used. After that pull off the sliders from both sides. they are not fastened in any way just use some force and they will come off. Fateman, thanks so much for your detailed photos and instructions. I had some clue about removal of the sunshade bow, but not about removing the clip from the bow. First you need to remove the sun shade bow from the rails. To do this you just pull one side of the shade towards the end so that the bow is positioned diagonally across the roof window. After that just push one side up to free the bow, then to the same with the other side and you have a bow freely hanging in the cabin. All you are left with is to install the repaired bow in to the car. Installation is quite simple, just reverse the disassembly process. Slide the curtain out. Remove the clip from curtain. Slide in the the bow on to the curtain. Put on the sliders. Install the bow in to the rails. Test the latch and have a beer or two because you have just saved 120€!