Hello Happy
I appreciate your devils advocacy on this subject, and I will try to clarify the situation in your given scenario. this will be a fairly long reply.
Let me first of all point out that I am not a legal expert, and what follows is my opinion and interpretation of the Law. It is however based on some web based research, and some experience of having to deal with trading standards where reasonable proof of fault was required.
I always recommend that anyone contemplating a serious warranty claim seek professional legal advice.
First of all, you need to get your head around the difference between a warranty and a guarantee. They are entirely different, and in many cases the words are incorrectly used and mixed up to describe the protection a consumer has, and who is liable and for what. The main difference is between a guarantee and a warranty is that a warranty is always enacted when a retailer sells a new product. it is the law and constitutes your statutory rights. A guarantee is not a legal requirement, but if one is offered it is a conditional contract.
A warranty is purely a statement like an MOT that only applies to the condition of a product at the point of sale. In other words it is the seller saying they believe the product they have sold is fit for purpose, and free from defects.
Using the MOT comparison, the issuing Vosa centre does not say the mot will not stand if you have the car service anywhere else; it is simply a snapshot in time.
So for that reason you do not have a 12month or even a three year warranty, the warranty lasts for the life of the product, or until it is sold on. In practice the longest a claim has been allowed under SOGA cases has been six years, but it does depend on the product and the degree of usage it has been put to.
Now if you do have a product failure and you wish to claim under WARRANTY, you must make the claim against the seller (no one else, not even a dealer who sells the same product). And you have to be able to prove** that the fault was either present at the time of sale, or that the cause of the fault was present at the time of sale.
** Proof in SOGA cases comes under civil law, where the test is based on the balance of probability (Criminal law requires proof beyond reasonable doubt). The burden of proof changes with time. In essence for the first 6 months, if a problem arises, then it is up to the seller show that on the balance of probabilities the problem was not present at the time of sale. After 6 months the burden falls on the owner to show that on the balance of probabilities the fault existed at the time of sale. Its not quite black and white!
The degree of liability that a seller has is judged on the merits of case, but in general it is discounted against the life expectancy of the product or 6 years which ever is shorter.
Using your scenario - If a fault becomes apparent it has to be shown that it (or its cause) was present at the time of sale. If it is disputed and cannot be resolved between you and the seller, then your course of action is through SOGA. Unless the service work could be shown to have contributed to the fault, then it has no bearing on the claim. Even if the service work may have affected the fault, the seller may still be liable for some of the cost. Only a courts ruling on the case could give you the final answer.
As for the service provider, they are only liable for the warranty on parts they supply, but they are also liable for the workmanship they provide (Workmanship is as much a product as physical part) so if they have contributed to a fault condition under SOGA they are liable for the damage they cause.
I have to agree with you that to avoid any dichotomy; you would have to only use the SELLERS service facilities. Avoid any other services, even if they are accredited to work on the same make of product. But it is not a legal requirement.
I will try to be briefer regarding Guarantees. As stated above a Guarantee is a conditional contract. It is a predictive statement that defines how the guarantor will act if certain conditions arise. In most retail cases (including caravans) the guarantor is the Manufacturer (not the seller or the dealer) In many ways it will give the appearance of working like the warrantee but it is not the warrantee as that only exists between you and the seller.
Being a contract, a guarantee has terms and conditions. For the contract to remain valid you must abide by the terms and conditions. If they require services to be performed only by subscribing dealers, then you must do so. If you wish to vary the terms of the contract you must get written permission, for example to have service or repair work carried out by a non-marques dealer.
Incidentally, if you make a claim under "warranty" (i.e. against the seller) the seller cannot refer the decision to the manufacture. The seller may request the manufactures opinion, but the manufactures opinion is not binding on the outcome, so even if a manufacture declines to reimburse the seller, that should not affect your rights under SOGA for the seller to make good a faulty product.
Not every retailer is aware of that, so don't be fobbed off.