Martin Kelly wrote: how/where I could get custom decals made for the more obscure markings.
Seems like Railtech are the go to company for custom transfers. https://www.railtec-models.com/showitem.php?id=3651
Somewhere on RMweb the proprietor did a full write up on how to draw up custom sheets and in what format to be compatible with his work flow. I've been meaning to have a go with this service foe quite a while but have been too lazy to draw up any sheets.
If you want to keep it simple B951645 may be a good vac. fitted candidate. This 1957 image shows the simple branding. By coincidence, I have a wet print of B951646 in identical condition but branded "London Midland" and the solebars are painted bauxite.