The train, the 10:35 from Paddington to Plymouth - diverted via Swindon due to Newbury works - came to a stand just after Chippenham shortly before 12 noon.  Two hours later it has just started moving back towards Chippenham as 5C78.  I am not sure what happened to the passengers.  I doubt it was really terminated at Swindon as it departed, as normal, as 1C78 on time.

It has nothing to do with an ability to manage annual leave. It's all down to the perfectly normal and valid situation whereby the entire railway workforce cannot be compelled to come to work on a Sunday. It's the Lord's day! Woe betide anyone who dares attempting to force staff to give up going to church. Strike, strike, strike! So if staff cannot be bribed into coming in with massive overtime payments then everything falls apart. Ain't railways brilliant.

Indeed:

Due to a speed restriction because of high track temperatures at Oxford trains have to run at reduced speed on all lines.
Impact: Train services running through this station may be delayed by up to 15 minutes. Disruption is expected until 18:00 23/07.

Interesting eyewitness account;

https://www.telegraph.co.uk/women/life/standing-joke-something-really-really-needs-done-state-trains/?utm_campaign=Echobox&utm_medium=Social&utm_source=Facebook#link_time=1528013684

14:38 Westbury to Bristol Parkway due 15:44
14:38 Westbury to Bristol Parkway due 15:44 will no longer call at Avoncliff.
This is due to a fault on this train.
Additional Information
To increase capacity on our Weymouth and Southampton services we are introducing 3 coach Turbo trains. Turbo trains require the fitting of local door controls to allow them to stop at stations with very short platforms. A Turbo train not fitted with these controls cannot call at Avoncliff, Dilton Marsh, Chetnole or Thornford.

Unfortunately it has taken longer than expected to fit the controls and the Turbo train being used on this service today has yet to be fitted.

If you at one of the stations the train is unable to call at please use the help point or contact National Rail Enquiries on 03457 48 49 50 and we will arrange for an additional stop on another train, or alternative road transport if another train cannot be stopped within a reasonable time.

We would like to apologise for the difficulties we know this has caused to your journeys recently.

...There is also a problem with other platform furniture being "unhelpful" for those waiting on the London bound platform.

...