Not sure of the exact area you mean Jagman but the Seaton Moor pit (No 1) worked up to outcrop in that area. There are a number of ancient workings pre-dating Seaton Moor in the outcrops of various seams, most of the names forgotten.
Hymac, the oxygen content quoted by Jagman is too low to support combustion and without knowing the cause of the deficiency (ie is it firedamp build-up or blackdamp) the possibility of explosion is difficult to determine. If it is firedamp there is the possibility of an explosive mixture being formed at the interface between the mine atmosphere and fresh air where the mix could be "just right" (5-14% methane in fresh air equivalent). The deficiency is, however, more likely to be due to blackdamp caused by the slow oxidation of coal and rotting of timber. The exploration of such places should never be encouraged - it was hazardous enough entering disused parts of working mines (with some ventilation still present) never mind places long closed. The re-opening of any abandoned coal mine is a serious undertaking, carefully planned and supervised by experienced persons. Even that can go pear-shaped - a local example was the attempt to re-open Haig's "Development" district in 1928 - 13 men, including the colliery manager and mines inspectors, remain down there to this day along with the man they went looking for!