In the meantime I have read "British Prisoners of War in First World War Germany" by Oliver Wilkinson. In it the author states: "In theory, as soon as a man was captured he was given a postcard to send home which... conveyed his name, number, regiment and nature of wounds (if any)." How soon any of these cards might reach the next of kin is unclear and notice he writes "in theory". No one in our family has seen any such card if indeed one was sent by my grandfather.
The book is an interesting read if you are interested in finding out more about how POWs were treated and how they coped with captivity.