Hello Craig,
I am sorry to read of your problem, however I cannot understand how this was not discovered any earlier? 12 months seems a long time! If it is obviously a manufacturing fault I am surprised the dealer has refused to correct it, so my suspicion is that there is some doubt about its cause and it may not be a manufacturing fault.
For everyone's benefit, it is better to avoid unpleasantness, so try to resolve it with the dealer, find out why they are refusing, and try to convince them otherwise. If that fails then you may wish to consider further legal action.
In any situation like this you need to be wary about costs, because if you intend to pursue it through your legal rights, it is unlikely you will be compensated for time and trouble related to pursuance of the issue. Your unrecoverable costs may be more than the cost to have it repaired.
Use written correspondence and keep all documents. If you have a phone conversation ask for a written confirmation of the details of the call.
William has identified an interesting document, and I think you might find the following link of interest also:-
http://www.dti.gov.uk/consumers/index.html
In a nut shell - your contract is with your seller, and they are obliged to sell satisfactory goods, and if the goods are faulty they have to put them right or compensate you, however and importantly whilst there is no fixed time limit on this obligation it depends on the nature and use of the product, if it goes to court the delay in bringing the problem to the attention of the seller will be considered in any judgment, along with normal wear and tear and actions of the owner that may have affected the condition.
Now you will note that I have referred to the seller rather than dealer, this is because your contract is with the who ever sells the caravan to you, and if you have used a finance package, then the finance house may be deemed the seller. Clearly you did not purchase it direct from the manufacture, so Swift are not obliged to you and should not be involved at this stage.
In cases like this where it is not clear cut it is important to establish the fault and its probable cause. You would be wise to employ an independent engineer to inspect and report on the problem. If the report supports your view, then you should send your seller a copy of the report and ask to have the fault fixed under the terms of the Supply of Goods and Services Act 1982 (Supply of Services).
If they still refuse, then you need to seek professional legal advice.