Hello again, intransient
I am fairly certain that the manufacture have in this case got their legal liability wrong.
The contract under which these "repairs" were undertaken was the Manufacturers Guarantee. That guarantee does not come into force automatically, you have to sign a document and it is returned to the manufacturer so the contract for the guarantee is between you and the manufacturer not the dealer.
This situation is reinforced when you took your caravan to the dealer, before the repair could commence, the dealer had to seek approval from the manufacturer to carry out repairs on their behalf and to their specification - not yours.
Further, it is the manufacturer that pays the dealer to carry out the repairs, which means the dealer is a subcontractor for the work.
With the exception that you agreed to let the manufacturer's choice of dealer to do the repair, You have had no influence on the contractual arrangements between the manufacture and the dealer in respect of the work carried out and payments or costs for the work.
This in my opinion has all the classic elements that demonstrates the manufacture in this is the main contractor and thus liable for the design and quality of the repairs carried out. They cannot absolve themselves of that responsibility with respect to you. The contractor is responsible in law for the actions and performance of their subcontractors, and they must ensure the works are carried out to a proper standard.
Do not forget that as I have expressed previously, the the use of the manufacturers guarantee does not prevent you from pursuing a claim under SoGA.
However I do think the way your problems are developing, you would be wise to seek a professional view that can review all the documentation and offer proper and legal advice.