Indeed, the hired in SWT▸ 158 is normally to be seen operating the alternate hour services between Swindon and Gloucester/Cheltenham Spa throughout the day. One of which is extended through to Southampton to form the evening Melksham service.
I enjoy using this particular unit as the 1st Class section is declassified!
It is indeed rather interesting that the "local" daytime Swindon -> Cheltenham service, which runs on the odd hours between the London -> Cheltenham trains, is entirely sourced with local trains arriving up via Chippenham into Swindon - with the 07:48 arrival from Westbury, and the 09:04 (?) arrival from Temple Meads. The latter is now comprised of 2 units, so that's 3 units for Stroud Valley use.
Seeing as so few of the 15x trains based in Bristol go to Swindon via Chippenham, and so many make an appearance at Gloucester / Cheltenham, one wonders why the sourcing is done from the south. But there is a certain logic, looking back at the discussion of the train on the TransWilts, in basing units that run a Salisbury / Swindon service at Salisbury. Hmmm ... perhaps that's too radical!
