The sliding door is found in the Heritage 560-4 & 640-ES models to close off the bedroom at the rear.
Extracted from the forums
I managed to find replacement parts for a very reasonable £2 or so from locks on line - as opposed to £53 from a caravan parts shop......
http://www.locksonline.com/acatalog/running-gear-infront-12032.html
http://www.locksonline.com/acatalog/housing-11871.html