My reading of bus and train timings for commuters from Frome / Westbury / Trowbridge / Melksham in to Swindon from December is as follows:

From and to Frome

06.47, change at Westbury, arrive Swindon at 07:50
07:05, change at Bath, arrive Swindon at 08:40

17:26 from Swindon, change at Bath arrive at 18:51
18:42 from Swindon, change at Westbury, arrive at 19:54

From and to Westbury and Trowbridge

07:02 from Westbury and 07:08 from Trowbridge, direct to Swindon at 07:50
07:31 from Westbury and 07:38 from Trowbridge, change at Bath, arrive at Swindon at 08:40

17:26 from Swindon, change at Bath, arrive at Trowbridge at 18:27 and Westbury at 18:34
18:00 from Swindon, change at Bath (4 mins), arrive at Trowbridge at 18:50 and Westbury at 18:59
18:42 from Swindon, direct to Trowbridge at 19:17 and Westbury at 19:24

From and to Melksham

07:17, direct to Swindon at 07:50
07:22 from Melksham Market Place, change at Chippenham, to Swindon at 08:10
08:05 from Melksham Market Place, change at Chippenham, to Swindon at 09:10
(this latter service leaves at 08:12 during school holidays)

16:56 from Swindon, change at Chippenham to bus, arrive Melksham at 18:02
18:00 from Swindon, change at Chippenham to bus, arrive Melksham at 18:56
18:42 from Swindon, direct to Melksham at 19:08