It can be seen that this is a relatively early Mk1 carriage, because the windows are flush with the metal bodywork. In BR service, this window design led to terrible rust problems in a surprisingly short length of time, particularly around and below the windows. Later builds of Mk1 coaches (and earlier builds that had their windows replaced by BR to counteract the problem) have a raised edge to the frame, it being an aluminium unit that overlaps the bodywork on the exterior. I believe the NVR has examples of both types in service in the Mk1 rake.
According to the Vintage Carriage Trust database, this particular coach was built in 1957, at York. It was preserved in 1987 on a section of track as the 'Railway Inn' restaurant, Whittlesea, Cambridgeshire. According to the same source, it was donated to the NVR in 1998 although it didn't arrive at the railway until November 1999.
In the most recent edition of 'Nene Steam' (No.98, Summer 2010) there is an appeal for volunteers, experienced or not, to help restore No.4794/Christabel back to running order. I believe there has been some interest in the project (although more volunteers will always be welcome), so hopefully the future for this coach is looking rosier. The condition of the bodywork gives cause for concern however - if the project progresses, repairs to the metalwork will almost certainly have to be undertaken in the workshop by the full-time C&W restoration staff. I love to see neglected things being restored, so I really hope this project generates the required support. I also hope it's given an authentic paint job!
