There is a danger of different issues being muddled. As I see it, there are 3 basic issues:
1) liability – is it enforceable? (Irrespective of planning permission).
2) if there is liability, is it on a commercial basis?
3) there is no appropriate planning permission for residential dwellings / caravans on the site.
The difficulty is that the issue of planning permission does not necessarily make the liability unenforceable. Even if the landlord is in breach of planning permission, he will still (normally) be able to create an enforceable agreement with a tenant on the site. Therefore, my view is that there is actual liability and it is enforceable.
As for non-commercial, I’d be more than confident if I was to represent the clmt.
Planning issues will rarely affect HB in these circumstances. There is a CD or two where this was touched on (can’t locate it for now).
Hope this helps.