Avril, I don't know if this will be of any help, but here goes!
If you had diptheria you would have been in an isolation hospital. There was such an institution on top of the mountain directly across the valley from Fochriw, beside the mountain road from Markham to Tredegar. I remember it well for as Kids we used to dare each other to run up and touch the walls!
Fochriw has a website, I can't think of the link at the moment, but just google it. You could also try
www.welshcoalmines.co.uk there are some pretty knowledgeable people there. There's a family tree section and also a general chit-chat.