Steve, useful info thanks very much, I'll try and source the book mentioned.
The wagons were unfitted but could be fitted with a temporary pipe for operation in a fitted train, particularly in later years. Brake vans were used anyway not only for carrying the guard, but also the attendant load inspector. The info about the headcodes is also extremely useful!
I've seen BR loading drawings showing conflats, Lowmac and Loriots acting as runner wagons. Quite often, there would also be bogie bolsters used to return the empty wagons (they often didn't run unloaded).
What does seem weird is the lack of photos of the wagons operating in an actual train. I assumed they'd be photographed, being specially diagrammed trains, but this appears not to be the case.
thanks