Thanks Cardiff, not got a lot to offer BUT local knowledge!
Brentor boy, if you see the geography of this area, you'll realise that it's all windy lanes and huge hills and mountains. If the injury was that severe that he died of them, given the depth they worked, it's unlikely that your man was moved that far - not massively familiar with how transport worked back then, but I doubt they would have put him on a coal train (or would they?), and I suspect a car/lorry/ambulance ride would have taken at least an hour to the nearest hospitsl - again this is dependent on whether your relative was in a condition to be moved.
My father still works in a mine in the area, and I know that he has to walk at least an hour underground until he gets to the coal face.
Again, not an expert in mining, but I'm fairly certain that the conditions even in the 1920's were fairly awful in terms of lighting, accesability, etc.
If you get the death certificate, let us know the place of death - if it's in Glyncorrwg (and assuming it's still standing), I'll get a photo for you