Hi, Tom, and welcome to Airforums.com! Ask the questions, and you will get asnwers to most anything you need to know.
You're in Essex, which isn't too far from London. London is the home of Can-Am RV Centre
, which is the only Airstream dealer in our part of the world, and thankfully they are a very good one.
For the owners and service manuals, I suggest you drop in and see them, or call: they have a good assortment of both of those in stock, and the prices I saw are less than what Mary quoted for Airstream's pricing, plus postage within Canada will be a lot less, too.
As to the worth of your trailer, there is a wide range of values, and so much depends on condition that one could only say that it's worth between C$2,000 and C$10,000. Just as an aside, if everything is original, it's not hard to spend ten grand just to upgrade it all!
How about posting some photos?
L-o-t-s of photos.