Above is an extract from my re-drawn map of the Ball Eye - Wapping map for the Matlock Book, which I sent to the editor on Tuesday, its preparation going ahead now. This shows the various soughs at Ball Eye, as far as I know. The site of the Flat Stone is way over in Holland's Mine (near the right margin of the crop). That was the point where the water from Ball Eye could be turned to go either to Masson Mills or Matlock Bath. My ambition in the 1970s was to get to the Flat Stone and then decide whether Matlock Bath or Masson Mills should be flooded. The continuation of the sough under the quarry was so dangerous that it stopped Lawrence Hurt (a really tough explorer) in c.1968 and when we turned up in 1970, it had collapsed more. Realistically, the best hope of regaining the Flat Stone would be to attempt Holland's Mine.
Incidentally, Matlock Sough is an invented name - I do not know the old miner's name for the Ball Eye end but the bit west of Holland's Mine was known as Hopewell or Hopeful Sough. Fountrabbey Level was just a made up name, should be Ball Eye Level, which is confusing because there were other Ball Eye Level - including the one partly cropped off the bottom right corner of the above map.
You are doing marvellous work with this digging at Ball Eye - keep it up!