Little help to be offered I'm afraid.
TOCs▸ are free to do as they wish with Advance Purchase fares. It seems that
FGW▸ are now only offering a ^22.75 Advance Purchase for that journey whatever date or time of day you pick, right up to the booking horizon. I've looked at a couple of booking engines and none seem to allow the ^22.45 Super Off Peak Single for the 2245 off Paddington.
Only saving I can see on that ^22.75 is an AP to Swansea at ^8.25 (on your chosen date of 13th Sept) plus the Anytime Day Single Swansea - Knucklas at ^12.00. Total ^20.25.
Railcard prices quoted and it would appear that the 0436 Swansea - Knucklas is subject to the 16-25 Railcard minimum fare of ^12.00 before 1000, which is a bit mean. I hardly call that train a peak service. But rules is rules.
