Hello Allyson,
Warranty is a slightly more complex issue than you allude to in your posting.
A warranty is a statement that confirms that goods conform to a particular standard. If it subsequently is proven that the goods do not comply with that standard, then the seller has made a false statement, and is therefore liable in law to repair, replace or refund the goods as appropriate. This is fundamentally what your statutory rights are under the Sale of Goods legislation.
There is no requirement in the warrantee that the goods must be serviced by a particular group of people (such as NCC approved dealers), and if there were, it would almost certainly be deemed as an unfair contractual clause, and therefore unenforceable.
What is expected is that servicing will be carried out, when and where necessary, and that parts and workmanship will be to an acceptable standard. It is up to the owner to decide how this will be achieved, and that may well be by using an NCC approved workshop.
The use of none NCC approved workshops therefore does not affect your 'warranty'.
The Warranty rests exclusively with the seller. This must not be confused with the Manufactures Guarantee that is an entirely different and separate matter.
Your contract to purchase a caravan is between you and the seller. In most cases the seller will not be the manufacture, so the manufacture is not legally obliged to you.
However, most manufactures recognise the fact that caravans are not easily transportable, and that by their very nature they may be used in places well away from their sellers location, so to help they have introduced a Manufactures Guarantee. As they are not legally obliged to provide this, it is therefore technically a gift to you from the manufacturer.
As it is a gift, the manufacture can impose a variety of stipulations on how the gift can be used. This may include things like: only use the manufactures dealers, the guarantee is non-transferable.
Under these circumstances the manufactures guarantee may become void if the clauses of the guarantee are not followed.
The picture can become even more clouded when you are offered an extended guarantee. These are simply an insurance policy, a fact that you should be clearly told when you are offered one. - and in my opinion not worth the cost of purchasing them.