Drivers require time converting to the Castle Sets as there are differences, i.e. doors for one thing. I don't know what Swansea to Cheltenham HSTs are supposed to be and was the information reliable. Short HSTs have been considered by TFW but seems to be dead in the water what with the extensive training required both with drivers and the length of training time required.
Which is a shame as short set HST's would be a good interim solution to TFW's stock shortages.