Ooh - if you missed the 18:42 at Swindon because the 17:30 was running late, your ticket will also be valid on the 06:19

More seriously, I have noticed (you'll see a case in point that happened to me) a tendency for a relatively relaxed view to be taken to enquiries such as yours which passes the buck on to the next person down the line ... you're then OK unless you happen upon someone who interprets the rules more harshly, or is so fare removed from the initial incident that he thinks you're trying to pull a fast one. "In Writing" is a good idea as was described in that other thread, which I will point out is NOT an
FGW▸ example.