AFAIK no sleeping cars were ever allocated to the Southern Region, although the BY is very likely an SR one as modelled by Hornby. It's presence may be explained by the need to provide heating for a guard riding in it; CCTs were not fitted with steam heating or through pipes for it and the BY has a stove in it. If this is the case a LMS or LNER 6 wheeled BZ would have done just as well.
A Van C, perhaps http://www.bluebell-...l/pics/404.html
I should have stated earlier that I model in N gauge.