Simon,
Cannot help you with a price but the following may be of interest. It is of course a calculated guess you understand as I have no connection with any of the people or brands mentioned below!
The model Keith Appleton was modifying was probably made by the Wuhu Brands Arts & Crafts Co. Ltd. in Anhui Province, China. They have a brand called Bowande which is their division for live steam models.
On the Alibaba website for Wuhu there is a direct link to their website http://www.bowandeusa.com.
On that website under dealers they list two in England:
Bowande Live Steam UK Ltd., Rickmansworth. They exhibit at exhibition in the UK.
The other dealer is Kingscale, Braunston, Northamptonshire. Kingscale is of course the trading name for live steam locos from Silver Crest Models.
Keith Appleton makes it clear in a disclaimer that the model he modified is not a Kingscale or Silvercrest model but was bought directly from the manufacturer in China and delivered to Blackgates Engineering.
Station Road Steam sold a 14xx “manufactured in China by Silver Crest Models” in 2017. It was new in 2016 . Their description is worth reading! Especially the warranty conditions!!
Interestingly all the pictures show a loco numbered 1466 and I am sure the GWR built more than one.
If you are so inclined Simon I will leave you to join up the dots!
John