And for today ... the first train is replace by a bus because, as far as I can tell, the line is needed to run a more important West Country express that calls at Westbury, with engineering works closing the normal line through Pewsey.
I am not sure if that was the case. The up train from Warminster is usually timed to be on the single line between 08:26 and 08:38. Today the Penzance train was slightly early and was on the line between 09:02 and 09:14.
The return service from Swindon is usually between Thingley and Bradford Junctions from 09:27 and 09:38.