Thanks to everyone who emailed in to help me identify the wagon. Pretty much everyone said it was a G.W.R. Siphon of some sort, and it turns out it is a type F. Now normally I'd groan - out of region and period, but have been doing some digging, and found one in a train of Thompson and Gresley vehicles...on the Eastern Region!
Prototype for everything it seems!
Until next time, thanks for all of your help chaps.