I welded up the steel walls and then clad them with OS board to give some extra strength.
The whole wall had to be built out to allow enough room for the roller bar and pocket door, it was then clad again in OS board to strengthen it up.
The match boarding will then be pinned to this to give the finish.
I've also decided that it would be better to have a internal glass window above the bathroom door to allow light into the bathroom, which along with its reletivly large size for such a small space should stop it from being like a lot of pokey caravan bathrooms. (90cm x 250cm)