Hello Jon,
If there is a manufacturing fault with the caravan, The sale of Goods cat is very clear it is the responsibility of the seller to effect a remedy. - Not the manufacturer.
To be able to claim under SoGA you must have to prove that on balance there was a manufacturing fault that was present at the time of sale.
There is no hard and fast rule that warranty only lasts for 12 months, in fact there is no fixed time limit, and in some cases a claim can be lodged against the seller for up to 6 years but it does depend on circumstances. where extends times are involved, the seller liability may be discounted against time.
Your seller may utilise Swifts experience but only in the capacity as a subcontractor. Swifts opinion in this matter actually has no bearing in this matter as your contract is with your seller not Swift.
If your seller has admitted that the symptoms suggests a manufacturing fault then they have tantamount admitted they are liable for selling the faulty parts under SoGA. Technically a verbal indication of liability should be sufficient, but it is even more compelling if it is in writing.
On the down side, roof lights are very vulnerable to damage from passing under low branches etc, so it becomes less clear that the damage is simply a manufacturing fault.
If it is a manufacturing fault, why did it not become evident earlier in its life? 4 years is quite a long time.
Given the passing of time, without prior evidence of the fault, the sellers liability is likely to be significantly discounted. Also given the innate vulnerability of roof lights to damage whilst towing, I think it would be difficult to give sufficient weight to the probability that the damage is the result of a manufacturing fault
I believe you would spend more in trying to prove a fault than any court would award in damages.It is probably more cost effective to simply replace them, and put it down to a life experience.