Hi Chris
I am disapointed that no one has replied, so here is what I know.
On the moor itself, few open adits go far. The exception is the Golden Dagger. A few years ago the adit was open in a sort of a way, but water up to your gills, dont know current situation. SRT entry possible to workings either side of a blockage, but be careful.
Most of the Devon stuff is around Tavistock and Gunnislake, plus of course Great Rock.
Tinminer is the one to ask about this.
My avatar is a poor likeness.