It's Cardiff City Away on the 4th of February. Here are the current train prices:
London Paddington 10:30 - 12:46 Cardiff Central
Cardiff Central 18:26 - 20:38 London Paddington
Current Non-Railcard price is £54.00 (1st Class Outbound, 2nd Class Return)
I advise booking this asap to avoid these cheaper tickets being unavailable.