Hello Carol,
I am not a legal expert, but I have had some success at resolving sale of goods issues that I have encountered.
It is vitally important that you have to accept that you must play fairly, Do not exaggerate issues, that might later become evidence in a legal claim. This is civil law and is judged on the balance of probability.
One has to accept that there is a greater risk of product failure which increases with age. With second hand goods the risk is increased because of the unknown previous use to which the product has been exposed.
With an item like a caravan, it is sad fact that they do wear out, and some age related issues will occur almost regardless of how much they have been used. Also establishing what would be a fair rate of wear and tear is difficult, so each issue has to be weighed carefully.
You have some inalienable statutory Rights; Traders are bound by Trades Description and the Sale of Goods, Acts. These require that goods, must be described fairly and be of merchantable quality, fit for purpose and free from material defect. The same basic concept applies for second-hand goods, purchased from a trader, but reasonable allowance must be made for wear and tear, and in some cases defects can be present, provided the purchasers attention is drawn to the fact. Armed with that information the purchaser can make an informed decision to purchase or not, and enter into a conditional contract that absolves the trader of responsibility for the notified condition. All traders are bound by these laws and they cannot avoid the consequences of them.
It is also the case that the purchaser of a second hand item must make all reasonable checks that the product meets their specification and must not assume anything - Caveat Emptor or Buyer beware. You can ask the seller a question and expect a reasonable answer do this before you purchase.
Warranties are in addition to you statutory rights, and they can be made conditional.
Now in relation to you posting, you purchased a second hand van in Oct 2004. You do not state how old the van was, but my guess, considering the purchase price. it would be about 2 years old at purchase.
You would have to review the contract of sale you signed at the time of purchase to view the terms of any warranty the dealer gave, If those documents state that damp ingress is excluded from the dealers warranty then you have little comeback. If you could prove that the caravan was faulty at the time of purchase, which resulted in the de-lamination you might have a chance, but obtaining such proof at this time would be very difficult. If it was not mentioned, then the dealer cannot absolve themselves entirely from considering repairs under warranty, BUT their liability will be diminished because of the age of the caravan.
Your concerns of damp became apparent after 5 months of ownership but you chose not to return the caravan to the selling trader to have the condition verified, (the dealer has been denied the opportunity to check the condition in a timely manner) so it is only your word that the condition existed at that time. I would guess that unless you have independent expert witness statements to the condition, the dealer has no due cause to act in your favour, and could justifiable claim they were blocked from assessing or correcting the condition, and because of the delay the condition has been allowed to deteriorate unreasonably.
In such matters there is a duty of care on both the seller and the purchaser, to act reasonably. In preventing the dealer from having the opportunity to inspect the problem at the time it was first noticed, you have seriously weakened your case, and realistically I think you will find it is a case of putting it down to experience. The dealer may choose to help you but that is in their gift and would be an act of good faith.
As I stated at the top, I am not a legal expert, and I would suggest that it would be essential that you seek professional legal advice before you embark on any kind of action.