Bonair 850 1981

Hiya folks
I'm a newbie from over the pond. I'm collecting a new (to me) trailer next weekend. It needs a whole heap of jobs doing and one of them is to replace one of the tension wires. Apparently it winds up ok.
How easy is it to do? And has anyone got any replacement wires over here in the uk or what gauge wire is used? Do I replace with the top up or down? Does the unit need to be jacked up?
I'm going to probably have to replace the roof or parts of it so have been looking at the renovations on the forum.

Thanks for your help folks.

Kim