Hi there,
thanks to this thread I managed to fix the lumber support in my Octavia today.
I had a different fault so I thought I'd post the fix here in case someone else has the same problem. I had the same symptoms as the other posts here. While stretching I heard the twang of a spring and suddenly the back support wasn't there anymore.
Without removing the seat I opened the back up as shown above. Luckily all springs and cables looked to be ok and in position. What had broken was the plastic T-shaped arm. (See crude addition to a photo I borrowed from above. The red line indicates the break) This arm holds the cable and when tension is applied brings the lumbar support forwards. I glued the piece back together. I then put the T-shaped arm back into place, did not lock into position just yet. It was awkward to get the cable back into the slot as it is in the picture. So I disconnected the lower wire from the snaps on the lumbar support for a little more movement. You need to get the metal at the end of the plastic cable sleeve lower than the slot in the T-shaped arm to thread it into place. Once that was done I locked the T of the arm into place. Snapped the bottom wire into the lumbar support and put the seat back together. Job done, happy out.