Hello Fiona,
I am sorry to read of your loss, and the difficulties you are now encountering. I sincerely hope that it can be resolved satisfactorily.
I hope that Dustydog can offer some help,
But my views on the matter is this:
Provided you supplied all the details of the site and its limited security measures and the insurance company accepted them, then they cannot refuse to pay out solely on the grounds of insufficient external protection.
You would have a very strong case if you have written confirmation from the company of their acceptance of the arrangement, provided this pre-dated the policy being issued. But as virtually all insures proudly tell us that phone calls are recorded for training purposes, even a verbal acceptance should be sufficient, provided the recording or the transcripts confirm the agreement.
That all sounds quite positive, but I have to be realistic and draw your attention to the possibility that you may have misinterpreted the policies, clauses or information from the company. This is why written details are so important.
I assume that you either obtained your policy through a broker or from a direct line. Which ever way you will have spoken to an 'agent' who as far as you are concerned represents the company and who speaks or writes with the full authority of the company.
If the company now states that an agent does not have the authority to vary the terms of a policy then their agent was wrong to imply that you could be covered. You would have a claim against the agent for misrepresentation.
If the company retrospectively changed the agents agreement with you, you should have been advised in writing and offered a full refund of any premium payments made or the chance to conform to the policy details.
If the company continued to accept your payments, despite the fact they knew of your pre-disclosed situation and as a result were not prepared to cover your caravan, that is Fraud.
Sadly It all hinges on whether the arrangement you thought you had was actually fully agreed and properly adopted by both sides.
On possible last glimmer of hope, even if the actual outcome was that your caravan not covered, but the communication's were ambiguous or not sufficiently concise, it may classed as an unfair contract.