As I arrive the Tuesday, my usual strategy is to get a morning train from Temple Meads to Westbury, then get the larger GWR connection to Castle Cary.
Much quieter service and I'm generally guaranteed a seat.
Booked it about this time last year, and cost me £7. This year I'm looking at the exact same service and it's £20 quid!!
I really don't want to get the direct train to Castle Cary as it's tiny and likely to be standing room only, but that's much cheaper in comparison.