Another mystery solved! I walked along the path from The Garden House to The Midland recently and wondered what the tunnel exits from the side of the hill were. I was wondering if they were part of the infamous "tunnel rats" network, but now it seems they were part of the mine drainage.
My parents contributed a chapter on the canals to a book in the 1970s called "The Historic Industries of Marple and Mellor". I should probably read it sometime; I might learn something.