Hi
Checked with a friend, there is a church with a graveyard in St Mellons aptly called St Mellons Parish Church:St Mellons Vicarage, Church Lane, St Mellons, Cardiff. CF3 5UP 029 2079 6560 - this is very close to Rumney, there is also another church in St Augustines, in Church Road, Rumney Cardiff CF3 3BA
Hope this is of help
EAC