Rear seat back cover fastening

  Anyone know how to re-fasten the covering on the rear seats? Hoovering out the boot of my other halves Fabia today I noticed that the fastening on the bottom of the left most seat (what you see from in the boot) was coming unfastened. It appears to be a zip, there are a couple of loose ends that are folded inside, but as soon as its touched it just undoes more and there's no fastener to pull along to re-fasten it. I'm worried that if I do anything more the whole thing will come undone.

Anyone had to do this