Neil
Regarding the China Clay wagon,you need a GWR Dia.013 wagon,Parkside do the 4mm version and I have some 7mm scale resin body kits.I could let you have one or two,for a reasonable price of course!
I've also found an interesting small loco for you,a Baguley-Drewry 0-4-0 std. gauge shunter(Petrol or Diesel engine)with a wheel base of 5'6" and 3'1" wheels.
Remind me before Thursday and I'll bring a Dia.013 wagon with me to the Brunswick,aswell as the Baguley-Drewry book.
Cheers
Tim