Most will want to change the whole backend as it may be easier, Try to find a 'smartcar' repairer in your area, they mainly repair plastics (smart cars) and are very good at hidden repairs.
------------- Doing as little as possible for as much as possible...
for a 2 year old van, personally I would have the rear replaced at a dealer, dont Baileys have a 5 year water ingress guarantee? if so it will NEED to be done properly to keep it.
it also depends on how long you want to keep the van, if your going to sell it on in the next couple of years, get it done properly, if you are going to keep it for the next 10 years, a repair wont affect the re-sale value too much.