Hi Betty, my great uncle James Buckley was killed in Tonhir Colliery, near Maesteg, in 1914 (his last shift before he was due to enlist!).
He was killed when a rock struck him on the head.
At that time (although I don't know how far back they used to do this), the mine would be inspected at fairly regular intervals, and cracks in the roof and walls would be noted by an inspector.
There was an inquest into James' death, and this appeared in the local newspaper (of which I have a copy), so there is a good chance your man may well have had his own inquest as well. It provided me with time of death, time he started his shift, the fact that his father was working at the same mine at the time (albeit on a different shift), etc.
If you can find the local paper, you may strike lucky