I used to have a pair of those W&U coaches, built up with a scumbled exterior finish. They started life, as Jon says, as part of GRS's G3 range. However, as is the wont of any business looking to recoup tooling costs (their body's are quite substantial resin castings the bogies in white metal) they, no doubt due to their diminutive dimensions 1:1, were also marketed as "freelance" products with 45 mm wheel sets for the 16 mm scale market.
One little tip if you get a set un-built Jon. The coach side moldings are ever so slightly "lopsided", as were the balcony ends too, end to end/side to side. Not uncommon with GRS products of this type. So be careful to check and correct this when assembling. I was told by the MD of the moldings company I used to use, CMA, that you cold tell if a pattern maker was right or left handed partly because of this sometimes noted "distortion" in a pattern affecting the final build.
If you get to see "The Titfield Thunderbolt" film the coach used there is one of the original W&U items. Max