Heading out of Birmingham, you'd have the best chance of a seat between 16:00 and 18:00 on the Cardiff Centrals. The Bristol Voyagers tend to be standing room only at these times. Depending what time you'd need to be in Birmingham for in the morning will determine your best option. There's a Voyager that starts at BTM (calls Cheltenham at 07:41 for an 08:30 Birmingham arrival), which will have only stopped at Bristol Parkway and is pretty light for the time of day - I've always managed to get a seat on that one.
Heading out of Birmingham, you'd have the best chance of a seat between 16:00 and 18:00 on the Cardiff Centrals. The Bristol Voyagers tend to be standing room only at these times. Depending what time you'd need to be in Birmingham for in the morning will determine your best option. There's a Voyager that starts at BTM (calls Cheltenham at 07:41 for an 08:30 Birmingham arrival), which will have only stopped at Bristol Parkway and is pretty light for the time of day - I've always managed to get a seat on that one.
Thanks all, very helpful. I tend to work slightly later into the evenings so would hopefully find a spot after the initial off peak rush. In the mornings it would be the 7.41 (great to hear it’s not too painful!) or even the 7.57 from Cardiff and 8.11 from Plymouth. Does anyone have experience of these services?