[Andymharrison]

I think this is one of those subjects where nobody knows the law from the guidance.

When we started carrying propane we were told various things from 2 cylinders only, up to 200kg and also 333l.

The 333l may come from a HSE document that says private individuals can carry an aggregate of 333l of dangerous goods as long as all containers are UN type. Many plastic containers aren't.

Next time ask them to call the police and see if they have a clue!