A 9m deep hole by the track side explored by ADEMCO. Marked AD-5.
[The given grid ref of ED50 0454197 4798961 seems quite close.]

Reference: ADEMCO, 2012 pp15-16
Entrance picture : ADEMCO, 2012 p15
Underground picture :
Video :
Detailed Survey : from ADEMCO, 2012
Line Survey :
On area survey :
Survex file :