Sorry to reactivate an old thread, but I had the same problem, couldn't find a solution in the archive so decided to post what I have done. My car (09 R57) also developed a gap at the rear left, where the top attaches to the body.
Inside the stitched edge is a thin, flexible plastic stiffener, with a hole in it backed by a thin stainless grommet. This attaches to a mount on the inside with a spring clip. With the hood pulled up at the back you get torturous access if you have long arms, but you do get the slack required to pull out the edge as in the first image. The grommet came off after removing the spring clip. I later found better access from the back seat! Note the grommet shows clear signs of having been heat-welded in an abortive repair.
I made a reinforcement to bridge the gap from a thin piece of 6061-t6 sheet, with a hole to match, shown in image 2.
I fixed this behind the plastic stiffener with 1/8" Al rivets, (image 3) then farted around for ages getting it back on the mount, referring to the other side to see how it all sits.
Image 4 shows the post you are mounting it on, with a rubber sealing strip removed for access. I would not have had to remove the strip if I was working from the back seat. The channel appears to be an water trap, the white plastic thing may be a drain.
Image 5 shows the spring clip in place.