I cannot definitely answer your question and I suspect that this is not the best place. You would be better off going to the newsgroup soc.genealogy.britain and asking.
There are plenty of experts there who can tell you exactly where to write. I think you have to write to the MOD in Hayes, Middlesex and show that you are next of kin. Alternatively, you will have to pay a researcher to visit the Public Records Office in Kew and check the medal rolls. Unfortunately, a lot of WW1 documents were damaged in WW2 and are still being microfilmed, so even that might not work.
Sorry I cannot help you further, but good hunting!