If the dealer won't repair it under warranty for some reason, then you should be able to purchase a new inner frame. If it's the same as the type that I've had dealings with, then this should be able to be removed by undoing 4 screws. They may be under plastic trim blanks. You won't have to replace the whole window, but as is traditional with anything connected with caravaning, it probably won't come cheap. I expect that you may even be able to then remove just the roller blind bit from the frame, but getting hold of a replacement item may not be very easy.