Thank you for your recommendations, though I feel you confidence in me is actually misguided, I do not have legal training, so my advice is purely how I interpret what I (think) I know.
However from my perspective, the rules about sale of goods do have some bearing on private sales. Underlying virtually all private sales is the Latin phrase caveat emptor or buyer beware.
This fundamentally means that the buyer must satisfy them selves that the product is suitable for their needs. They must do this by putting all relevant questions to the seller before the transaction is completed.
The seller is obliged to answer all questions honestly, to falsify an answer could be construed as fraud, however the seller is not obliged to offer information that the potential purchased does not specifically ask for. So for example, if the caravan had only one wheel but the buyer did not ask 'how many wheels does it have? The seller is not obliged to offer the information. The buyer should inspect the goods, and ask about anything they are not sure of.
When the purchaser is satisfied with the answers they receive and have agreed the price, on completion of the transaction, the original seller is relinquished of all responsibilities to the sale, unless the buyer can show that the seller deliberately gave an untrue statement about the product.
Now the test of the situation is governed by civil law, and as such the judgement is based on the balance of probabilities of how the disputed action was conducted. As a rough guideline, if the transaction was completed in such a way that if a third party had observed the transaction, and was satisfied it was conducted honestly then it is likely the transaction will stand and the seller not held responsible for any subsequent issues.
However in this case, as you have been made aware of the buyer's dissatisfaction, I think it would be prudent to gather as much information as you can about how you conducted the sale. Copies of any documents that you may have used, and your service reports etc. Construct a log of the events as your recall they happened, and any of the questions your buyer actually asked, along with the answers you gave and keep this ready.
As others have already written, don't do anything else, until the buyer actually issues the summons. If you do receive a summons, then that is the time to seek professional legal advice such as the CAB or a solicitor.
Based on the information you have supplied, I doubt that you will be deemed responsible for the cost of repairs, mainly because the caravan is of a substantial age, and any purchaser should be aware of the natural deterioration that does occur, and also the buyer has waited a substantial time before bringing the issue to your attention. But it very much depends on the details of how you conducted the sale.