Glover is right on the mark about the H vans. Any layout depecting the 1950's to the mid 1970's should have lots of them. I'm suprised that a kit has not been available.
SSM provide 30 ton brake vans and wooden opens and ex GN 12 ton vans and sliding door vans can be adapted from Parkside Dundas kits, what we need are H vans, Bulleid opens and CIE cattle wagons to convincingly model the CIE loose coupled era from the 1960's to the mid 1970's. Add a 20 ton brake van and some older vans and you have the 1950's covered also.
It would also be nice if any new wagons produced could be converted to 21mm guage without major surgery. Best of luck with the project Liesley.