Hello Sharon,
At this stage i cannot give you much help but if you can find out your fathers birthdate you should be able to get a copy of his birth certificate, which will give his fathers name, a possible address and a start with your search. Since he was born in Mountain Ash it is possible the birth may have been registered in either Pontypridd or Merthyr. Have you any idea regarding his mothers maiden name or know if his siblings were younger or older
Best Wishes,
Tallpaul.