I don't know how regular they'll be at leaving Ffos y Frân, but they'll have to be regular at the Cardiff end in order to squeeze between the passenger services at Queen Street. Looking at the public timetable, the only southbound gaps in the service between 0700 and 1900 are as follows:
0706, 1116, 1251, 1516, 1841
Northbound: 0704, 1044, 1109, 1519, 1848, 1858