The Nottingham-Cardiff is only given 18 minutes in the public timetable to turn around at Cardiff, which whilst isn't the shortest, it is short for a long distance service, and very short for performing a reversal at Cardiff Central given its restrictive signalling (aka carry on going West, cross all the tracks, then come back in).
It is often turned back at Newport, and at the very last minute therefore not giving everyone waiting at Cardiff time to get an eastbound train to make the connection, much to our delight. If one sees the down train is running late from Gloucester; one would be wise to get to Newport quickly as an insurance.
Hopefully the re-signalling of Cardiff at Christmas should solve this. They will then be able to arrive directly into Platforms 0, 1 or 2 to return back North.