I certainly didn't expect to see this! Less than half an hour after it passed South Marston on 29 June 2018 with the 5Z19 10:03 North Pole IEP Depot to Highbridge Loop crew training run, 800001 was back again, clearly having gone nowhere near Highbridge! It had reversed at Swindon, and was now running as the 5Z21 11:55 Swindon to North Pole IEP Depot.