Hello Mark,
This is another example of dealers not reading and understanding the details of the manufacturers detailed instructions, and then passing on incorrect advice.
Strange as it may seem, all though you purchase your caravan from your seller, the seller is only liable for the warranty established under the Sale of Goods Act. They have no liability or authority under the Manufactures guarantee, which is a separate contract between the caravan owner and the caravan manufacturer.
(When you have any repairs done under a manufacturers guarantee, the dealer is not under your instructions, but acting as a sub contractor and under instructions from the manufacturer.)
The caravan manufacture gift you the 'manufactures guarantee' but it is dependent on you purchasing a caravan. Because the guarantee is not a legal requirement (unlike the warranty), manufactures can and do place a number of clauses into its terms and conditions. These can be as tight as they wish to make them, and may include inflexible criteria for certain actions such as services.
The dealer was wrong on two counts. Fundamentally the seller has no title to vary the terms of the manufacturers guarantee, and the actual advice was faulty.
I suspect you were given this advice verbally, and whilst in UK consumer law a verbal agreement is in theory at least as valid as a written document, in practice it is very difficult to prove unless you have independent witnesses to the conversation who understood by what they heard that an agreement had been made. Otherwise its your word against someone else's. Its then down to who is more believable.
On the other hand, if you can have that conversation corroborated by an independent witness, or it was given to you in writing from your dealer, then they have no leg to stand on, and in theory you could make them liable for any loss of use of the manufacturers guarantee due to missed service.
Just as an example, I purchased a second hand car from a main dealer. As part of the deal I agreed to take out a mechanical breakdown insurance policy on the vehicle. I was given a policy booklet, which had a multipart carbonless policy application form in the back which the dealer filled in and stamped all the pages and took the top two copies, one for them selves and one to be sent off to the insurer. The copies already had a policy number printed on them.
It was just as well that I did, because it had a major mechanical fault. I duly contacted the insurance company, and was surprised to be told that no such policy existed, even though I had the receipt for payment and a policy booklet with a policy number. The dealer at first denied all knowledge of the policy and declined any liability.
Having got the customers copy of both the sales receipt and stamped insurance policy I pointed out to the dealer that I had paid for the policy (as itemised on the receipt) but the insurer had not received any payment. Now either it was a genuine mistake that the dealer had not sent the documentation or it was fraud. They suddenly started to take notice when I started to talk about trading standards and sale of goods.
The repairs were carried out (something like