Hello again Alfred,
If you genuinely believe the fault was a manufacturing problem, then you may have a case against the seller under SoGA, BUT the onus is on you to establish that "on the balance of probabilities" (the criteria for civil cases)the fault was present, but not necessarily manifested, at the point of sale.
One should also Bear in mind that there is a duty on the customer to take reasonable care to preserve and use a product correctly, the older a product is the more difficult it is to show correct care has been used, or that no activity has occurred that might have caused or exacerbated the apparent fault.
If you can get an independent caravan engineers report to describe the fault and its likely cause, and if that confirms your suspicion, then you may wish to consider presenting that information to your seller with a request for the fault to be put right under the requirements of the SoGA.
I must warn you though, if the seller resists the claim, and you choose to pursue it through the small claims courts, you should take professional legal advice before proceeding.
I expect you will be told by your adviser that each party is usually responsible for their own costs (i.e. The engineers reports, legal and court fees), and that if the claim is upheld, the award may be reduced due to the age and part of of the products normal life expectancy that has been used up.
You should also be aware that any remedy under SoGA is only to return the customer to the same position as if the fault had not occurred.
My heart says you should stick up for your rights. But my head tells me that may be difficult to establish the problem was present at the point of sale and in this case it may cost more to pursue the case, than to simply have the cover replaced.