If you know how, you can still find ways to travel, and I did make Salisbury for 11:45. But no thanks to
GWR▸ who failed to point me at the alternative. There's a new Sunday bus service from Melksham to Trowbridge, and I was able to catch the x34 at 09:51 from the Market Place in Melkshm to the back of Sainsbury's in Trowbridge where it terminates, and then walk the six or seven minutes to the station. Then catch the train that was running - at 10:54 (1F09)
* No suggestion from GWR of this option
* No directions from the bus stop to the railway station (not sure why it terminates there rather than at Trinity Church which is by the rail station)
Ongoing train - 4 cars 158 which has "full" though I has able to have a spawling, shifting youth move aside from his second seat. Others were standing
Return train ... I didn't know when I would be through ... caught the 14:20 back (1F20), which was a 4 car 158 again, even more stuffed that the one I had caught down. Just a single "open" seat in the front carriage, from which I persuaded the German family of 3 at the table for 4 to move their stuff onto the luggage rack. I suspect it was busy because of the cancellation of the previous service.
15:07 into Trowbridge. Next Melksham train (which may not have been running anyway - I didn't check) a long way off - so was the next bus (16:20 - hourly except for a couple of 90 minute gaps I happened to hit) and back home at around 5. There is a shelter from the rain by Sainsbury's but little seating or shelter from the sun - not really suitable as a bus terminus.
So - the journey WAS possible and made, but no recommendation of this routing in any way that I found. Google "Melksham to Salisbury" probably would have susses it out for me ...