Russ don't confuse cost with dryness. Many new caravans have damp that's found at the first, second, third, fourth and fifth annual service.
have you tried the caravan company
https://www.thecaravancompany.com/dorset/
this branch weren't very good when I got mine from another branch but as it had manufacturers warranty there was less of a problem.
I think you will struggle to get a dry van for £700 so you may need to consider how much damp you will accept
macafee2