Hello Stuart.
There is a technical and important difference between a warranty, and a guarantee.
Your warranty is automatically established when you make a new retail purchase and it consists of your statutory legal rights. It is supported by the Sale of Goods Act (SoGA) and it only exists between you and the person or organisation that sold the caravan to you. That might be a dealer or a finance house if you used one.
In essence the seller warrants that the goods sold are in merchantable conditions and fit for purpose. If there was a fault with the product at the time of purchase the seller is legally liable to correct the situation. Some faults do not become apparent until some time after the sale, and providing their is an overall probability the fault was present at the time of sale, then SoGA will support a claim for up to life expectancy of the product or 6 years (which ever is the shorter)from the point of sale. However the age and use of the product will affect the degree to which the liability is held against the seller.
Warranty work can only be undertaken by the seller, though they may subcontract the work to another organisation for example the manufacturer with no additional inconvenience or expense to the customer.
What you are more likely referring to is the manufacturers guarantee. This is totally separate to the warranty, and because it is not a legal requirement, the manufacture can establish what ever terms and conditions they see fit. You should read the terms and conditions supplied with your guarantee and abide by them.
If you wish to vary the terms of the guarantee you must first of seek written permission from the holder of the guarantee, in this case the manufacturer.
Not withstanding the above situation, most manufacturers will continue to honour their obligations under the guarantee provided that the necessary services and inspections have been carried out by an agent that is a dealer for the manufactures. IF in doubt always check with guarantor.