Not sure whether it had any bearing on the engineering works, but there was also a major power failure on that route on Saturday night (23/7).
This had the effect of halting every train in the Swindon area, just as I arrived in at just after 2100 on the 2001 Cheltenham Spa-Westbury service. Our service was cancelled from Swindon, and the passengers on it including myself sent on our way very efficiently by
FGW▸ staff to Melksham, Trowbridge and one all the way to Southampton by various taxis - very good effort on the their part, THANK YOU

Just before we left, they had diverted one Swansea-bound
HST▸ via Gloucester, and were preparing to arrange alternative transport for many other passengers.
Finally, I was also on the 0730 Westbury-Swindon, 0820 Swindon-Weymouth and 1756 return services on Sunday (24/7). I'll let grahame do the detailed report, but I can confirm that it went very well indeed.