For those who don't know the layout at Swindon that should be a cross-platform connection - no stairs or lifts involved.
Less than 10 seconds from being release from the train at Platform 3 across to the departing train - admittedly at the very front - of platform 2.
Real Time Trains shows the 17:36 leaving 4 minutes late anyway ... not making that up but rather dropping another couple of minutes on the run to Westbury. Hints from the
RTT» log suggest that the further loss would have been due to intermediate station calls lasting longer than scheduled due to numbers getting on and off through the end doors of a 153, and perhaps jamming just inside the carriage as people pick there way around others standing, or ask people to move up.
No onward connections from Westbury were compromised, and the return trip of unit started on time from Westbury; we know Swindon have advised holds based on main line connections for Gloucester trains and perhaps there's a case of having 5 minutes on some of the Westburies. To be fair, that's more important to connect from London trains than from South Wales, though passenger survey data does show a noticeable connectional flow.