Got good info on The Great War Forum, Thanks for the link Bugle Boy.
mmm45 is correct they are wearing hospital clothes, the cloth is called blue ticking and was used widely in hospitals all over the world as it was usually made from cotton and could be boiled to remove nastys.
still-looking is also correct the Giessen camp was used for propaganda purposes, among other things it didn't have top bunks making the sleeping quarters a lot more pleasant.
I also got a link to a very good free on-line book
http://digital.library.upenn.edu/women/mcclung/out/out.htmlwhich is a very good read, not finished it yet but from it I learnt that Giessen camp had a separate unit for "lungers" , (TB section) a good chance so that is where this photo was taken.
Also in this book is the difficulty with clothing and uniforms the POWs had as manmack said.
A good result and I learnt loads, thanks to each of you for your help.