Interestingly I used Google maps to give me a route from Temple Meads to Melksham today leaving at 13:45 (to allow time to get off the train etc).
It recommends the 14:00 Paddington train to Bath and the 14:20 D3 bus from Bath to Melksham; arriving at 15:08.
This routing is also offered by traveline.info.
Neither mentions the 13:49 train.
That's the bus I took ... not sure where in Melksham the 15:08 timing is for; I used a station estimate.
I took the 13:49 train as a piece of "defensive scheduling" - next available train and giving me 14 rather than 7 minutes between the train and bus in Bath. I have just looked back, and see that both the trains were a couple of minutes late yesterday - I suspect I would have made it off the 14:00, with a bit of a dash - I saw that pulling in on the viaduct as I was queueing to join the bus.
Perhaps the information desk at Bristol Temple Meads should use Google or Traveline