Hello Iain.d,
Have you tried Hobby tech for brass tube? As a builder of things I have sometimes found that they have what I need in stock.Good hobby shops are thin on the ground here in W.A. as you have probably found . I would like to see a photo of the coach when you finish it if Tony allows. I am an EM modeller ,GWR ,LNWR andMidland all pre WW1. Doug.