To answer the queries above in part at least...
The air last Tuesday was a bit odd, one of the party couldn't get his lighter to stay alight but there was a slight draught coming down Whalf Old Sough from Crimbo vein and I didn't experience any symptoms of bad air. However, if people are concerned then maybe try a spot-test with a lighter before heading into Meccano passage?
As for a lead vein, I didn't have time to go through the Bung so I couldn't comment, I'll look on a future visit. I did however note surprising amounts of galena in the sediments which I can't imagine the miners having left, so have these washed into the area post-abandonment?
BTW Ric, our handiwork from last year is standing up nicely!
Follow the horses, Johnny my laddie, follow the horses canny lad-oh!