Yes, looks like No 5029 ("Nunney Castle") will be stopping to take water at about 6:20 this evening, mikey.
This class of locomotive is a very appropriate choice to work a train through South Wales. They were introduced to work the crack expresses from London to Swindon, Bristol, the West Country and South Wales They were the GWR's most powerful locos until their more powerful cousins the "Kings" were introduced. The Kings were a development of the Castles with a larger boiler and were among the most powerful steam passenger locos ever used in the UK. However, they could not work beyond Plymouth into Cornwall because they were too heavy for the Royal Albert Bridge at Saltash so Cornwall was the domain of the Castles when a powerful express loco was needed. More info about Nunney Castle can be found here:
http://www.iconsofsteam.com/locomotives/nunney-castle#about
Dai Woodham's yard at Barry was responsible for you being able to see Nunney Castle in working order today. At least 200 locomotives, including No 5029, were rescued from there by heritage railways across the UK. Ironically it was because steam locos were quite difficult to scrap that led to their preservation. Woodham brothers bought redundant wagons as well as locomotives from BR but because the wagons were easier to scrap he concentrated on them first. Enthusiasts saw the potential at Barry and Dai was happy for them to pay a deposit to secure the locos (which he sold to them at scrap value) and store them in his yard until the full purchase price was paid.
By the way, you won't find too many "Train Spotters" around these days. As we've grown older we've nearly all morphed into "enthusiasts" !!! :-)