Hello Craig,
There are two types of answer to your question depending on what the specific problem is.
If the product is not fit for purpose, or is not of merchantable quality, the remedy lies entirely with the seller. These conditions are covered by your statutory rights as defined in the sale of goods acts.
see: http://www.dti.gov.uk/consumers/index.html
These statutory rights exist exclusively between you and your seller. These are set in law and cannot be changed. Please note that the seller is the organisation to which you pay your money, so if you have used a finance house then you deal with them.
The second area is the Manufacture's guarantee; These usually afford extended cover or special terms defined in the small print. Here the manufacturer offers the guarantee, and if you agree to it, it forms a part of the contract of sale. If you wish to retain your rights to this guarantee, you must abide the limitations and stipulations made in the contract. Things like service intervals or the routine inspection of certain parts. Failure to keep to the agreed schedule can void part or the whole of the manufactures guarantee.
Frequently the manufacture will stipulate that service work should be carried out by one of their dealers. This is a grey area, and similar limitations in the automotive field have been judged as unfair through the courts. Provided the work is carried out to the standard required by the manufacture or accepted industry standards, then, it may be performed by competent service centres not controlled or franchised by the manufacture.
Jones Venning offers independent assessments of the competencies of caravan workshops, and in general any of their approved workshops should be able to carry out the basic caravan servicing on any make of caravan.
http://www.jones-vening.co.uk./index.php
For clarity, you should contact your manufacture to seek permission for service work to be carried out by a JV approved workshop near you.
If the manufacture refuses, then you should seek professional advice to challenge the terms of your contact. (Trading Standards, and a Solicitor or CAB)