After looking at Britannia colliery I noticed that you have Bargoed Brithdir colliery listed at being in Mid Wales. This colliery was the house coal pit of Bargoed colliery and all three shafts (the 2 main Bargoed steam coal shafts and the house coal or Brithdir shafts were on the same site). There location is :
51.6898902619465,-3.223508872856