I largely concur with Surfer on this one, but I think you may need some additional explanation. There are basically three forms of protection you may have:-
Statutory Rights under Sale of Goods Act or warranty
When you purchase something you automatically have certain statutory rights as defined and supported by the Sale of Goods Act. These rights are exclusively between you and the seller, and the seller cannot avoid the liabilities these rights give you. Surfers quotes are fine and they explain what you can expect from SoGA. Warranty work is the responsibility of the seller alone.
Manufacturers Guarantee
Parksy's points relate to the manufactures Guarantee. In most cases when you make a new retail purchase, you are also offered a manufacturers guarantee. This must not be confused with your statutory rights, they operate in an entirely different way. Sometimes the guarantee is automatically started and sometimes you actually have to sign to accept it. Manufactures are not obliged to offer any guarantee, and as such it is governed by different rules. Even though money is not part of the arrangement, the guarantee is as much a contract as any other, so if you accept the manufactures guarantee to take advantage of it you must abide by its terms and conditions. Usually one of the T&C's is a requirement to have the caravan serviced or inspected in accordance with a strict schedule. If you do not keep to the schedule, you are in breach of contract and that permits the manufacture to decline any claims. Usually guarantee work can be carried out by any dealer authorised by the manufacture not just the seller
Breakdown Insurance or Extended Guarantee
In some cases when you make a new retail purchase, you are also offered an "Extended Guarantee". This is not strictly a guarantee at all, it is an insurance policy, and you pay a premium for the cover it provides. This is a contract, and to take advantage of it you must abide by its terms and conditions. Breakdown insurance work can be carried out by any person authorised by the insurance company.