Nowadays, with more seats on a HD HST▸ set, they just combine South Wales and Bristol services into one service every hour reversing at Parkway and Temple Meads stopping at Bath before taking the Wessex main line down to Westbury to join the B&H▸ to Reading.
Not strictly true. Though I can remember back in 2008 that they did the above. And the trains were VERY packed out, with lots of people standing. This must have been when there was also engineering works on the mainline between Bristol Parkway and Swindon. I did think it was bit daft combining the two services, and there also being only one service per hour from Bristol stations to London.
Earlier this year when services took this route due to planned weekend engineering works, there were two services per hour from Bristol Temple Meads. The xx:30 services would run via the Trowbridge & Newbury. Whilst the xx:08 services would run fast/non-stop to Swindon via Bristol Parkway, then onto London. The services from South Wales would still run via the usual route.
I can even remember one weekend back in 2008, when at some parts of the day there were actually three trains per hour between Bristol Temple Meads and London Paddington. The additional third service would stop at most(if not all) stations between Westbury and Reading, with journey times of about 2 hours 40 minutes.