The reference to the article about this level (properly called Bonsall Leys No. 2 Level) is:
Flindall, R.B., and Hayes, A.J., 1971. The Adit Workings on the North Side of the Via Gellia. P.D.M.H.S. Bull. Vol. 4 Pt.6 pp429-450.
I wrote that article and have recently been compiling a report on the history of mining in that area.
The 150 feet deep turn (called by the spar miner Cyril Maddocks in the 1950s the Thunder Shaft) was on Flixen Rake but it was always blind (even when it was first sunk) and there is nothing to be discovered really in the level at all. I wrongly stated in the article that the raises no doubt went in stages up through the toadstone to the surface but they would be blocked by collapse. In fact, the raises lead only into short trial levels just below the toadstone.
The only air flow in this long level seems to be that originating from a three inch wide open rift in the right wall of the level about 15 feet before the final forefield. The sound of falling water can be heard faintly in the distance (or at least it could in the 1970s), which is surprising because the overlying toadstone keeps all that strata very dry. There must be some sort of fault to allow the water to penetrate. There is a moderate airflow into the rift in summer. I cannot understand why the miners did not investigate this as it could lead to some sort of vein or cave. This point is about 250 feet below the surface of Bonsall Leys.