Keith, I found the below comment on landrovernet.com, surely you have read yourself up on this subject? If not, then I hope this is useful info. Not clued up on Dinitrol products myself so I could be misinformed here.
Internal, you can use DINRC800,this will convert the rust to a hard, stable organic compound.
External I would apply a very thin coat of ML3125 cavity wax to the chassis as it contains rust inhibitors and will act as undercoat for the underbody wax 4941(black) or 4942(brown)
For wheel arches and high abrasion areas then use 447 in place of 4941,4942 but still apply the ML3125 first.