Is the Shaft that you mention to the right of Gauther Fold?
If so,then your level would be in a few 100yds before reaching the shaft,going by the section of the upper seams,i would imagine the Shaft to be an Air Shaft for ventilation,as an Incline would have been a simpler and cheaper option.
Mr Mike,any chance of us seeing the list of seams on the left of your BGS map,i am wondering if the Old Colliery in the village is Shackleton Colliery working the Little seam.